neutron: set connection string in neutron.conf

it's not recommended to put the connection string in plugin specific
config file [1].

[1]:
https://github.com/openstack/neutron/blob/master/etc/neutron.conf#L534

Change-Id: I3c9d7fe3d34385185f483b509dadf7e2ba920a45
Merge "neutron: set connection string in neutron.conf"
Merge branch 'master' of https://github.com/openstack-dev/devstack