Install bridge-utils only if needed

The package bridge-utils is useful only if we have nova-network.
If we use Neutron, we don't have to install it.

This patch provides a condition "neutron_enabled" to check before the
installation of the package.

Fix bug #1176087

Change-Id: Iee2ecbf2023ab211d67cb9bfeb3dbc5b4bb3eb89
Merge "Install bridge-utils only if needed"