Restore GARP by default for floating IPs

Restore the default value of the send_arp_for_ha configuration
parameter to 3. This parameter was set to 0 by default to
prevent kernel crashes due to bug 1273386.

Change-Id: Idbdc6ff687734776ede45127878af6ad9b8b3ca1
Closes-Bug: 1317834
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 "Restore GARP by default for floating IPs"
Merge "Remove hard dependency on novaclient"