The 2.49 microversion for the os-volume_attachments API
only changes the POST action to attach a volume with a tag.
But we should make sure 2.49 is backward compatible on the
GET, PUT and DELETE methods for the same API.
This makes VolumeAttachmentsSampleV249 extend
VolumeAttachmentsSample and also removes duplicate code
by adding the _get_vol_attachment_subs method.
In addition, VolumeAttachmentsSampleV249OldCinderFlow is
removed since it didn't really work properly (it was missing
a few stubs) and was redundant with the test_tagged_attach_volume
unit test found in nova.tests.unit.test_compute_api.
Change-Id: I757576d3474997c49c67745fb245122fc2c6decc
e3c089bf1d