Adds "list hosts" test case - Cinder

This submission adds a new test script in volume admin api catogery,
so as to verify list hosts functionality in cinder API. Also added
required support functions in the corresponding files of both JSON/XML
interfaces.

Implements blueprint: cinder-grizzly1-blueprints-implementation

Change-Id: I72cdcb0f562a2706e400bfd6245a69c8368c2c2f
Merge "Adds "list hosts" test case - Cinder"