Add skip method for volume test with the glance service

In some tests, it try to create volume with image
 "VolumesV2ImageMetadata",
 "test_create_from_bootable_volume",
 "test_bootable_volume_backup_and_restore",
so it should be check Glance is available before test.
If all tests in class is associate with glance, I used skip_checks,
and otherwise, I used testtools.SkipIf

Change-Id: I2302699f98b77b0ccad551ef11c4655544b3f827
Closes-Bug: 1675700
3 files changed