labs: fix neutron admin password

In the controller node's nova.conf, neutron_admin_password was set to
NEUTRON_PASS instead of the correct password. Fixed.

Also set neutron_admin_username and neutron_admin_password via dynamic
methods (to the same values they had before).

Change-Id: I69f1dbc699b9cf3252a77c1e2f085c814ef6292f