Add handling of 401 error from keystone (systests)

Sometimes after reverting VMs snapshots keystone client
can't get authorization token due to 401 unauthorized error.
We should handle this exception in tests and try to redo
keystone authorization.

Change-Id: I499855a6b13eacee4894a837d3170b9f550cf74d
Closes-bug: #1365376
Merge "Add handling of 401 error from keystone (systests)"