Add cluster-template-show command to CLI

Implements: blueprint python-savannaclient-cli

Change-Id: Ib024e5f00174c459e1289207bada42d3258520bd
Refactor display of node group template

Change-Id: I1a81448658b3966c5ee212a905be264d58a4915f
Implements: blueprint python-savannaclient-cli