Storwize/SVC: Fix races in host-related functions

In the Storwize/SVC driver, initialize_connection checks if a host
object exists on the storage, and if not, creates one. Similarly,
terminate_connection checks if there are any more mappings for this
host, and deletes the host object if there are none. Consequently, there
exist races between two initialize_connection calls, two
terminate_connection calls, or one and one.

The easy solution here is to use locks. Because locks are files on the
local machine, this implies that all cinder-volume processes managing a
given Storwize/SVC contoller run on the same machine.

Change-Id: I62cd992e7150e7d16f1a2b2be6ef61c64b638858
Closes-Bug: #1274123
Merge "Storwize/SVC: Fix races in host-related functions"