Description of problem:
After renaming a volume we must reinitialize the volume lease, since
sanlock does not allow querying or acquiring a lease without the correct
lease name.

Steps to Reproduce:
1. create a 4.0 dc
2. create a vm with a single disk
3. create two snapshots: s1 and s2
4. cold merge s1
5. upgrade the dc to 4.1
6. cold merge s2

Actual results:
Following error is raised and the merge fails:

2018-01-21 15:23:48,241+0200 ERROR (tasks/3) [storage.TaskManager.Task] (Task='d1823d98-2e55-4a70-82f6-26532a76ee57') Unexpected error (task:875)
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/vdsm/storage/task.py", line 882, in _run
    return fn(*args, **kargs)
  File "/usr/lib/python2.7/site-packages/vdsm/storage/task.py", line 336, in run
    return self.cmd(*self.argslist, **self.argsdict)
  File "/usr/lib/python2.7/site-packages/vdsm/storage/securable.py", line 79, in wrapper
    return method(self, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/vdsm/storage/sp.py", line 1862, in prepareMerge
    merge.prepare(subchainInfo)
  File "/usr/lib/python2.7/site-packages/vdsm/storage/merge.py", line 178, in prepare
    with guarded.context(subchain.locks):
  File "/usr/lib/python2.7/site-packages/vdsm/storage/guarded.py", line 102, in __enter__
    six.reraise(*exc)
  File "/usr/lib/python2.7/site-packages/vdsm/storage/guarded.py", line 93, in __enter__
    lock.acquire()
  File "/usr/lib/python2.7/site-packages/vdsm/storage/volume.py", line 1516, in acquire
    dom.acquireVolumeLease(self._host_id, self._img_id, self._vol_id)
  File "/usr/lib/python2.7/site-packages/vdsm/storage/sd.py", line 475, in acquireVolumeLease
    self._domainLock.acquire(hostId, lease)
  File "/usr/lib/python2.7/site-packages/vdsm/storage/clusterlock.py", line 409, in acquire
    "Cannot acquire %s" % (lease,), str(e))
AcquireLockFailure: Cannot obtain lock: u"id=aafed42c-8cc1-4710-ac79-0e9c55420675, rc=-227, out=Cannot acquire Lease(name='fe81c224-210e-4735-b4b6-ffe890581b01', path=u'/rhev/data-center/mnt/rich-nfs-server2.usersys.redhat.com:_home_storage_sd1/aafed42c-8cc1-4710-ac79-0e9c55420675/images/ae9ed879-ae1b-47e4-bcd1-8551972761c4/fe81c224-210e-4735-b4b6-ffe890581b01.lease', offset=0), err=(-227, 'Sanlock resource not acquired', 'Lease resource name is incorrect')"


Expected results:
Cold merge to pass.