Sync our version of the interprocess lock

The oslo version gained a exists function and
merged in the commented code we had so we can
go ahead and merge that into our source tree
and remove the comment (since it's not useful
anymore).

The update is from lockutils.py, oslo incubator
commit hash e3b52f20367e18.

Change-Id: I858a4412d03c48c4ef1c2506edf6334c66ded3f0

taskflow/utils/lock_utils.py

@@ -377,7 +377,6 @@ class _InterProcessLock(object):
         try:
             self.unlock()
             self.lockfile.close()
-            # This is fixed in: https://review.openstack.org/70506
             LOG.debug('Released file lock "%s"', self.fname)
         except IOError:
             LOG.exception("Could not release the acquired lock `%s`",
@@ -386,6 +385,9 @@ class _InterProcessLock(object):
     def __exit__(self, exc_type, exc_val, exc_tb):
         self.release()
 
+    def exists(self):
+        return os.path.exists(self.fname)
+
     def trylock(self):
         raise NotImplementedError()