neutron: change default credentials for nova notifications

By default, we should use "admin" tenant to get an admin token and make
the VM connectivity working in Nova.

Using Nova user will fail since Nova (services tenant) can't get an
admin token.
neutron: revert some changes for nova notifications

It seems we need to use "services" tenant to send notifications to Nova
API, using Nova user.
Merge pull request #481 from enovance/nova-notifs

neutron: revert some changes for nova notifications