iproute2: overwrite existing bridge if necessary

When iproute2 tries to add new network over existing bridge, brctl raises an
error: AssertionError: ["device test-network already exists; can't create
bridge with the same name"] while ifcfg handles that situation without
a problem.

I changed iproute2's addBridge to ignore this error.

I've already wrote a test for this case http://gerrit.ovirt.org/#/c/30578/

Change-Id: Iac194bf62afecf76300d6807c0bc458b9c40048b
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30657
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>