BZ #1110773 - ML2 l2population is missing from plugin.ini config

https://bugzilla.redhat.com/show_bug.cgi?id=1110773

Add a param for l2_population config to compute and networker.
Include l2population mechanism by default in
controller/neutron-server.
Merge pull request #290 from cwolferh/bz_1110773_l2_population_true

BZ #1110773 - ML2 l2population is missing from plugin.ini config