Add scale thresholds to split etcd and k8s-masters

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).

contrib/inventory_builder/tox.ini

@@ -11,7 +11,7 @@ deps =
     -r{toxinidir}/test-requirements.txt
 setenv = VIRTUAL_ENV={envdir}
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
-commands = py.test -vv #{posargs:./tests}
+commands = pytest -vv #{posargs:./tests}
 
 [testenv:pep8]
 usedevelop = False