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
Create a common function for method _parse_network_vlan_ranges used
by plugins.

The _parse_network_vlan_ranges method does the same thing for the
linuxbridge, ovs, and hyperv plugins. Create a common function for
the plugins to use instead. This paves the way for improving vlan
range verification (see #1169266) in one place.

Fixes Bug #1177428

Change-Id: Ie8c20807e9146dd9c8bc011dd3a4dc10ec871e0b
Merge "Improve checking of return values for functions in linuxbridge agent plugin"
Merge "Fetch routers ports for metadata access from DB"
Merge "Create a common function for method _parse_network_vlan_ranges used by plugins."