Allow connecting instances to an existing bridge

[Saviq]

It may be desirable to have the user tell us which bridge to hook up the instances to.

This is related to #118 but rather than setting up the bridge with the external interface, we'd let the user do it and tell us which bridge to use.

Caveat:
We'd not be giving out IPs at that point so need another way to find about the IP address.

[Saviq]

Duplicate of #118