Add initial cluster-* commands to CLI

list - list clusters
show - show defails of a cluster
delete - delete a cluster

Change-Id: I2469ca52c87878885cb6917df494052f2b2b600c
Implements: blueprint python-savannaclient-cli
Add initial node-group-template-* commands to CLI

list - list node group templates
delete - delete a node group template

Change-Id: I6b0bac7c585ea90c8d8f0511f7d612487449f982
Implements: blueprint python-savannaclient-cli