Move SSHPool into ssh_utils.py

utils.py is loaded by almost everything, but SSH
code and imports (paramiko) are only needed by certain
drivers.

Split this into a separate file to reduce overhead (and
things that can break) for commands like cinder-manage.

Partial-Bug: #1348787

Change-Id: I46896f2bd1fd0de2aedde8e87e255398e5bc3171
Merge "Move SSHPool into ssh_utils.py"