Replace ssh connections in san.py with a connection pool

[Nirmal Ranganathan]

Replace def _connect_to_ssh(self): to use a ssh connection pool instead of a new connection for each call.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/15255

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/15255
Committed: http://github.com/openstack/cinder/commit/b864a08b2aa7e7ba953dd4afce2c3042ba963ae7
Submitter: Jenkins
Branch: master

commit b864a08b2aa7e7ba953dd4afce2c3042ba963ae7
Author: Nirmal Ranganathan <email address hidden>
Date: Thu Nov 1 23:05:35 2012 -0500

    Adding a SSH Connection Pool.

    Adds a connection pool using eventlet.pools
    which works well with greenthreads. Adding new
    parameters ssh_min_pool_conn and ssh_max_pool_conn.

    Updating the _run_ssh method in san.py to use the
    connection pool and cleaning up the existing
    single connections, also added retries.

    Fixes bug 1074185

    Change-Id: I90dd89ffc025d09fc6ad060c4273508103b85456