This replaces KUBE_MASTERS with KUBE_CONTROL_HOSTS because of [1]:

```
  The Kubernetes project is moving away from wording that is
  considered offensive. A new working group WG Naming was created
  to track this work, and the word "master" was declared as offensive.
  A proposal was formalized for replacing the word "master" with
  "control plane". This means it should be removed from source code,
  documentation, and user-facing configuration from Kubernetes and
  its sub-projects.
```

[1]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-cluster-lifecycle/kubeadm/2067-rename-master-label-taint/README.md#motivation

This variable was added as KUBE_MASTERS_MASTERS. That's probably a typo.
Remove the redundant `_MASTERS` suffix. Also, document the variable in the
help message.

inventory_builder creates hosts.yaml file with hostnames like "node1",
"node2", etc. Even if specifying override_system_hostname=false, the
output of "kubectl get nodes" shows those hostnames ("node1", etc.)
without using actual hostnames.
To solve this issue, this adds an option USE_REAL_HOSTNAME to get
actual hostnames when creating hosts.yaml file instead of "node1", etc.

Change-Id: Ifd7dd7ce8778f4f1be2016cae8d74452173b5312

Change-Id: Ia3f30a1216b3ea063cd72c839ef6dff753cf10c6

by making clusters with fewer than 3 nodes have only 1 etcd node

hosts are ordered dictionary
remove ansible_user from inventory file

- fix order of entries when the new yaml file is created
- fix group in case there are no hosts in it

* rewrite inventory script to create inventory file in YAML format

* minor fixes to inventory script

* change requirments for the inventory script

* fix error with delete host in inventory.py script

* minor fix

* add to inventory.py script ability to indicate ip ranges

* add test for range2ip function for inventory.py script

some fixes

* add negative test for range2ip function for inventory.py script

Related to https://github.com/kubernetes-sigs/kubespray/pull/3684

This does not address per-node certs and scheduler/proxy/controller-manager
component certs which are now required. This should be handled in a
follow-up patch.

Also adds calico-rr group if there are standalone etcd nodes.
Now if there are 50 or more nodes, 3 etcd nodes will be standalone.
If there are 200 or more nodes, 2 kube-masters will be standalone.
If thresholds are exceeded, kube-node group cannot add nodes that
belong to etcd or kube-master groups (according to above statements).

Includes tox support for running unit tests.
Small note added to getting-started guide for using
inventory_builder.py

Also adds manual-only unit test.