ceph-volume: PVolumes.get() should return one PV when using name or uuid

ceph-volume: PVolumes.get() should return one PV when using name or uuid

It is possible to get duplicated pv entires from the 'pvs' lvm command.
If we're using PVolumes.get() with either pv_name or pv_uuid we can
safely return a single PVolume object. However, if we use pv_tags with
PVolumes.get() we must still raise a MultiplePVsError if many pvs
are found that have the tags, because they are not guaranteed to be the
same pv, as would be the case with pv_name or pv_uuid.

Fixes: http://tracker.ceph.com/issues/24784
Signed-off-by: Andrew Schoen <aschoen@redhat.com>