Fix block eventlet threads on rbd calls

Fix block eventlet threads on rbd calls

Commit Ibaf43858d60e1320c339f2523b5c09c7f7c7f91e caused new problem with
cross thread communication. According to Python documentation, code can
lead to a deadlock if the spawned thread directly or indirectly attempts
to import a module. python-rados spawns new thread to connect to
cluster. So I removed new spawning new thread to connect to rados. All
long-running operations calls whith python-rbd are still implemented in
native Python threads to block eventlet loop.

Change-Id: Ic9971254102914080383b63cd2807e36213dd6eb
Closes-Bug: #1401335