Add ha docs

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>

docs/ha-mode.md

+HA endpoints for K8s
+====================
+
+The following components require a highly available endpoints:
+* etcd cluster,
+* kube-apiserver service instances.
+
+The former provides the
+[etcd-proxy](https://coreos.com/etcd/docs/latest/proxy.html) service to access
+the cluster members in HA fashion.
+
+The latter relies on a 3rd side reverse proxies, like Nginx or HAProxy, to
+achieve the same goal.
+
+Etcd
+----
+
+Etcd proxies are deployed on each node in the `k8s-cluster` group. A proxy is
+a separate etcd process. It has a `localhost:2379` frontend and all of the etcd
+cluster members as backends. Note that the `access_ip` is used as the backend
+IP, if specified. Frontend endpoints cannot be accessed externally as they are
+bound to a localhost only.
+
+The `etcd_access_endpoint` fact provides an access pattern for clients. And the
+`etcd_multiaccess` (defaults to `false`) group var controlls that behavior.
+When enabled, it makes deployed components to access the etcd cluster members
+directly: `http://ip1:2379, http://ip2:2379,...`. This mode assumes the clients
+do a loadbalancing and handle HA for connections. Note, a pod definition of a
+flannel networking plugin always uses a single `--etcd-server` endpoint!
+
+
+Kube-apiserver
+--------------
+TODO(bogdando) TBD