Use public gateway for Trove if neutron is enabled

Fix Trove to use the PUBLIC_NETWORK_GATEWAY if neutron is enabled.
This is needed since in this case the guestagent is able to reach
the devstack host only via the public PUBLIC_NETWORK_GATEWAY and
not via the private NETWORK_GATEWAY.

Change-Id: I61ee707e16d0222e7f479369e769cbe7ef2de7d7
Merge "Use public gateway for Trove if neutron is enabled"