Fix neutron credentials in nova config file

Currently, neutron_admin_username option is not set, so it
defaults to 'null'. This breaks nova-api and nova-compute
when they are talking to neutron-server.

Closes-Bug: #1253043

Change-Id: Iec07a6e6e99f65fdb759c6820f66342f10080ac2
Merge "Fix neutron credentials in nova config file"