This commit moves the L2 agents (Linuxbridge and OVS) into the
ML2 directory, while at the same time also moving the ML2 server
bits into toplevel directories. It also moves the configuration
files and unit tests. We also move the l2pop RPC mixin while
here as well.
Partially-Implements: blueprint reference-implementation-split
Signed-off-by: Kyle Mestery <firstname.lastname@example.org>