Remove libvirt legacy LVM code

The Change-Id Ib36b962971fd1f66ea9a0818e91fec59e118e686 fixed an
issue which could have caused volumes no longer being able to be
cleaned up due to it's reliance on the instance_name_template
configuration option.

There was code in place to handle both disk formats which are
the legacy format (volume name prefixed by instance name) and
the new one (volume name prefixed by instance uuid).

This commit removes the legacy format which was due to be
removed from the code.

UpgradeImpact - this removes support for really old lvm names that
could have been created in Havana clouds. That means there is a
possible failure to cleanup of lvm volumes that existed uniterupted
since Havana.

Change-Id: I28497a650fbd012643080951f7d86ab8780fd59c
Merge "Remove libvirt legacy LVM code"