Default to linuxbridge and openvswitch drivers for ML2.

Since the addition of ML2 port-binding, the linuxbridge and openvswitch
drivers are required to be loaded when running with ML2. This small
patch adds their loading into ML2 into devstack.

Fixes bug 1220743

Change-Id: I97c5f4e0e4af59766e0084ed3b2dea2843cb33bf
Augment instead of override extra ML2 options.

The existing ML2 code overwrote extra options set in localrc with
defualt values in some cases. This fixes it so it no longer does
that and instead adds to rather than overrides those values.

Fixes bug 1222854

Change-Id: Iafdaad7d4253f1b61e8a214c50adaf7599a641f2
Merge "Default to linuxbridge and openvswitch drivers for ML2."
Merge "Augment instead of override extra ML2 options."