Check cached SSH connection in PowerVM driver

The PowerVM operator code caches an SSH connection to the hypervisor
which can become invalid if the connection to the hypervisor is not
terminated cleanly, e.g. the hypervisor is rebooted while the compute
node is connected to it.

This code checks an existing SSH connection object to see if it's
transport is still alive and if not, re-establishes the connection
before attempting to run SSH commands on the hypervisor.

Also added some basic unit tests to cover the new check_connection
function in nova.virt.powervm.common.

Fixes bug 1177104

Change-Id: I08079cf0d9e60e1d8902d32d684d979b06f7f287
Cleanup for test_create_server_with_deleted_image

Adds a cleanup for test_create_server_with_deleted_image so that
the test resets the test image to the active state after the test
finishes. Otherwise other tests which use the same image after
this test can fail.

Change-Id: Iedd039af54d2233d1afbc7fe2d21d1b3594c47b1
Merge "Check cached SSH connection in PowerVM driver"
Merge "Cleanup for test_create_server_with_deleted_image"