add auto creation of ovs bridge

Allow auto-creation of an ovs bridge (somewhat inline with auto-creation of a linux bridge). Throw an error if 'ovs*' is specified as a bridge and ovs is not installed.
IMHO, we should move that test to the creation code. Imagine the following scenario:

- someone is creating the OVS bridge from within a container (yes, that's possible :-))
- the OVS tools are not installed in the host
- they're trying to add a container to an OVS bridge
- ... then this check will bite them.

What do you think?