vdsm: packages: enforce network service

current implementation enforces network service only if bridge plugin is
enabled. While in ovirt-engine-3.3 the bridge is optional.

new implementation perform this unconditionally in the packages plugin.

Change-Id: I96ca90aa975c92439ba33309381462a052e44c5e
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>