Add Nova flavor constraint for SaharaNodeGroupTemplate

Add nova.flavor custom constraint for
SaharaNodeGroupTemplate resource, to validate
the flavor's existence.

Change-Id: Ia5abf6da88eb2f701978bcc637a417706831ed0f
Abstract rpc 'create_stack' call mock method

Abstract 'create_stack' rpc call mock method for 
test_api_cfn_v1.py.

Change-Id: I94b117bf2d142ff1a67affec503d9cfdb8a558cc
Merge "Add Nova flavor constraint for SaharaNodeGroupTemplate"
Merge "Abstract rpc 'create_stack' call mock method"