support persistent mac addresses on OVS bridges on RHEL

This patch adds support for RHEL-derived systems (RHEL/CentOS/Fedora)
for setting the persistent MAC address of an OVS bridge via the MACADDR
setting in the interface configuration file.

Without this change, when an administrator provides MACADDR in the
interface configuration file that address will be set in ifup-eth using
the "ip link set" command.  While this appears to work, any operation
that updates the OVS configuration will cause the MAC address to revert.

Persistent MAC addresses must be set using ovs-vsctl.

Signed-off-by: Lars Kellogg-Stedman <lars@redhat.com>