networks: activate connection properly and idempotancy for setup

Created by: SunilMohanAdapa

When network-manager restarts, it leaves the existing connections intact. So, active the newly created connection to make them work.

Only add a connection for a device if a connection with that name is not already associated with that device.

Closes #1045 (closed).

Minimal testing was done to ensure the results are better than what we have now. May check again later with full disk image but I think the patch can be merged.