Use service role for neutron instead of admin

When creating the account for neutron to use in keystone,
give it a service role instead of an admin role so it isn't
overprivileged with the ability to create and delete tenants.
Also set the Neutron policy.json file to allow the Neutron
account to administer Neutron.

Closes-Bug: #1344463
Change-Id: I86b15cfcffe549654c28f425c2bcf99403ac10bc
Merge "Use service role for neutron instead of admin"