Fix error when identity file is missing
Fix error when identity file is missing

When the ssh_keys mistral environment was not present or missing the
'private_key' variable the RunValidationAction would fail with
UnboundLocalError.

This commit makes sure the identity_file variable is assigned before
being referenced in the finally block.

Change-Id: I60507fb0589127ca7dd9e8490f14edf558e994e6
Closes-Bug: #1620476

openstack/tripleo-common,openstack/tripleo-common