ensure linuxbridge dependency is installed on RHEL

Package['neutron-plugin-linuxbridge'] is defined in
plugins/linuxbridge.pp but is missing in agents/linuxbridge.pp.

If agents::linuxbridge is used directly, the package is not defined.
This changes fixes the situation on RHEL.

The problem has already been fixed for OVS in
changeset Iedf8a607915f65b7a8cf7af99749b9b65cba7bd9

Change-Id: Ic4e15825e4f2d1f2ce2c34d2fdb01c8f6b19d491