TestCase to check set/get/unset flavor extraspecs

Adds functions in flavors_client.py to set/get/unset extra specs for
a flavor. Script by the name test_flavors_extra_specs.py is added so
as test the actions of set/get/unset extra specs for a flavor.

Change-Id: I1b57e8434fdef28676bdc4068760fe83054fb4f8
Implements: blueprint nova-grizzly1-blueprints-implementation
Merge "TestCase to check set/get/unset flavor extraspecs"