Feature request: Add support for multiple bridges

[HRJ]

I want to allow multiple users to create unpriviledged containers on the same host. And I would like each user to be assigned a different bridge, so that each user's containers can be restricted to a sub-net.

The `/etc/init/lxc-ent` script seems to be hard-coded for one bridge. If it could read a list of bridge names and create them automatically, it would be great.

[HRJ (harshad-rj)]

Btw, the access to the bridges can be controlled via `/etc/lxc/lxc-usernet`. So that is already in place. This request is just to be able to create bridges.