Remove hard dependency on novaclient

The nova notification patch introduces a hard dependency on
novaclient when it is a runtime-configurable dependency. The
import from novaclient should be conditional on the
appropriate nova notification options being enabled in the
config.

Change-Id: I2ef4bfa4d53afc7e8c800ad8e2a8737e117af238
Closes-Bug: #1321352
Merge "Remove hard dependency on novaclient"