Provide Neutron password in compute metadata

overcloud compute node makes requests to Neutron API
and requires quantum_admin_password option of nova.conf
to be set (it is defined in nova image element like
quantum_admin_password={{neutron.service-password}}).

Without this, booting of a user instance in overcloud
fails, because nova-compute service can't authorize
requests to Neutron API.

Change-Id: Ie726d0c3d54abc6c24a45fde3f5af03fd2cf9e37