support configuring bridge interfaces on EL-based distros

I'm not too familiar with Debian/Ubuntu-based distros, so didn't implement this for the Debian ::osfamily case.

Under EL, you can specify the parent bridge device in the child device's config.  But under Debian/Ubuntu, you have to specify the child devices in the config for the parent, I think.  Not sure the best way to handle that.

Happy to implement it, though, if someone else can help come up with a sane way to do it.
We definitely need it for our setup.  I'll see if I can get my hands on an Ubuntu VM and implement it for that, too.