Document commonly used Kargo vars

docs/vars.md

+Configurable Parameters in Kargo
+================================
+
+#### Generic Ansible variables
+
+You can view facts gathered by Ansible automatically
+[here](http://docs.ansible.com/ansible/playbooks_variables.html#information-discovered-from-systems-facts).
+
+Some variables of note include:
+
+* *ansible_user*: user to connect to via SSH
+* *ansible_default_ipv4.address*: IP address Ansible automatically chooses.
+  Generated based on the output from the command ``ip -4 route get 8.8.8.8``
+
+#### Common vars that are used in Kargo
+
+* *calico_version* - Specify version of Calico to use
+* *calico_cni_version* - Specify version of Calico CNI plugin to use
+* *docker_version* - Specify version of Docker to used (should be quoted
+  string)
+* *etcd_version* - Specify version of ETCD to use
+* *ipip* - Enables Calico ipip encapsulation by default
+* *hyperkube_image_repo* - Specify the Docker repository where Hyperkube
+  resides
+* *hyperkube_image_tag* - Specify the Docker tag where Hyperkube resides
+* *kube_network_plugin* - Changes k8s plugin to Calico
+* *kube_proxy_mode* - Changes k8s proxy mode to iptables mode
+* *kube_version* - Specify a given Kubernetes hyperkube version
+* *searchdomains* - Array of DNS domains to search when looking up hostnames
+* *nameservers* - Array of nameservers to use for DNS lookup
+
+#### Addressing variables
+
+* *ip* - IP to use for binding services (host var)
+* *access_ip* - IP for other hosts to use to connect to. Often required when
+  deploying from a cloud, such as OpenStack or GCE and you have separate 
+  public/floating and private IPs.
+* *ansible_default_ipv4.address* - Not Kargo-specific, but it is used if ip
+  and access_ip are undefined
+* *loadbalancer_apiserver* - If defined, all hosts will connect to this
+  address instead of localhost for kube-masters and kube-master[0] for
+  kube-nodes. See more details in the
+  [HA guide](https://github.com/kubernetes-incubator/kargo/blob/master/docs/ha-mode.md).
+* *loadbalancer_apiserver_localhost* - If enabled, all hosts will connect to
+  the apiserver internally load balanced endpoint.  See more details in the
+  [HA guide](https://github.com/kubernetes-incubator/kargo/blob/master/docs/ha-mode.md).
+
+#### Cluster variables
+
+Kubernetes needs some parameters in order to get deployed. These are the
+following default cluster paramters:
+
+* *cluster_name* - Name of cluster (default is cluster.local)
+* *domain_name* - Name of cluster DNS domain (default is cluster.local)
+* *kube_network_plugin* - Plugin to use for container networking
+* *kube_service_addresses* - Subnet for cluster IPs (default is
+  10.233.0.0/18). Must not overlap with kube_pods_subnet
+* *kube_pods_subnet* - Subnet for Pod IPs (default is 10.233.64.0/18). Must not
+  overlap with kube_service_addresses.
+* *kube_network_node_prefix* - Subnet allocated per-node for pod IPs. Remainin
+  bits in kube_pods_subnet dictates how many kube-nodes can be in cluster.
+* *dns_setup* - Enables dnsmasq
+* *dns_server* - Cluster IP for dnsmasq (default is 10.233.0.2)
+* *skydns_server* - Cluster IP for KubeDNS (default is 10.233.0.3)
+* *cloud_provider* - Enable extra Kubelet option if operating inside GCE or
+  OpenStack (default is unset)
+* *kube_hostpath_dynamic_provisioner* - Required for use of PetSets type in
+  Kubernetes
+
+#### DNS variables
+
+By default, dnsmasq gets set up with 8.8.8.8 as an upstream DNS server and all
+other settings from your existing /etc/resolv.conf are lost. Set the following
+variables to match your requirements.
+
+* *upstream_dns_servers* - Array of upstream DNS servers configured on host in
+  addition to Kargo deployed DNS
+* *nameservers* - Array of DNS servers configured for use in dnsmasq
+* *searchdomains* - Array of up to 4 search domains
+* *skip_dnsmasq* - Don't set up dnsmasq (use only KubeDNS)
+
+For more information, see [DNS
+Stack](https://github.com/kubernetes-incubator/kargo/blob/master/docs/dns-stack.md).
+
+#### Other service variables
+
+* *docker_options* - Commonly used to set
+  ``--insecure-registry=myregistry.mydomain:5000``
+* *http_proxy/https_proxy/no_proxy* - Proxy variables for deploying behind a
+  proxy
+
+#### User accounts
+
+Kargo sets up two Kubernetes accounts by default: ``root`` and ``kube``. Their
+passwords default to changeme. You can set this by changing ``kube_api_pwd``.
+