Support Router Advertisement Daemon (radvd) for IPv6

Install radvd if the l3 service is enabled.

Partially implements blueprint: neutron-ipv6-radvd-ra

Change-Id: I749b31dcaad072a35e5921579b50f1ca451b1383
Use Mongo DB as the default backend

With the gate upgrade to Trusty, Marconi should now be able to run with
Mongo DB at the gate.

Change-Id: I7bfe25d42c9429606ee209860685077806eb6756
Merge "Use Mongo DB as the default backend"
Merge "Support Router Advertisement Daemon (radvd) for IPv6"
Merge commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/18/112018/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch