Adds create_from_volume test cases

  For tests.test_volumes,
  added 3 test cases:
     create_from_volume
     create_from_volume_with_glance_metadata
     create_from_volume_failed_on_cloning
  refactored an internal api:
     _create_volume_from_image

  The reason not to test a non-exist volume as source_vol is
  the uplayer(volume api) has already checked and ensured the
  source_vol exists

Change-Id: I9c345bc29cae31f1a22236a4849f13d7289ea49e
Merge "Adds create_from_volume test cases"