Fix passing keystone token to neutronclient instance

Neutron client expects token to be passed as token= argument, while
neutron-metadata-agent passes auth_token= instead. This effectively makes the
client to authenticate against keystone each time it's instantiated. In
neutron-metadata-agent case, it means 'each time a client sends a metadata
request.'

The issue results in high cpu utilization on keystone side when simultaneously
invoking multiple nova instances with cloud-init.

Change-Id: I2a31f9c0c3cfa915975ecc53d71168a3895528d8
Closes-Bug: 1274487
(cherry picked from commit 3799efe792700e7736666c40cc42c265eee09230)
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable