Move RBD calls to a separate threads

Move RBD calls to a separate threads

RBD is a python binding for librados which isn't patched by eventlet.

Making long-running tasks like removing big (~100GB, ~1TB) volumes
blocks eventlet loop and all cinder-volume service hangs
until it finished when rados_connect_timeout is disabled. It makes
cinder-volume services unavailable for a while.

This patch moves all rados calls to a separate python thread which
doesn't block eventlet loop.

Change-Id: Ibaf43858d60e1320c339f2523b5c09c7f7c7f91e
Closes-Bug: #1401335