change time.sleep to use loopingcall

We don't want to block the volume manager from servicing
requests.  So this patch changes our use of time.sleep
to use loopingcall wait, which uses eventlet greenthread
sleep.

Change-Id: I13a1e4932e24ff5f09e35b8baa7c0fd5410388b6
Closes-Bug: #1286285
Merge "change time.sleep to use loopingcall"