network: Fix bridges for el6 and ifcfg

ifcfg can't ifup or ifup bridges unless /usr/sbin/brctl, that is
provided by bridge-utils is installed (Fedora19 does not install
it by default).

ip link del dev X type bridge is a not supported operation on
2.6.32 kernels that are found in el6 distros.

This patch fixes both things while keeping it compatible with
newer distros.

Change-Id: Ib798a5581191d835f68bd4324c2182ff28cab4a6
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>