Fetch routers ports for metadata access from DB

Bug 1176963

There is no need to retrieve the ports from the NVP backend, so
a DB query will be enough.

Change-Id: If3529cc5694434e8cdc9d21a4043f94212146d2d
Improve checking of return values for functions in linuxbridge agent plugin

Fixes bug 1167780

Changed return values for some ensure_* functions to make them more consistent.
Now all ensure_* functions return None on failure and return the entity that was
created within the function when successful. Made changes to the unit tests to
reflect the changed return values.

Change-Id: Ib015ee7cee50bae5d91a4e109e7381519c1e14f7
Merge "Improve checking of return values for functions in linuxbridge agent plugin"
Merge "Fetch routers ports for metadata access from DB"