Add neutron settings

These settings will hopefully configure devstack-gate to run with
neutron.
    DEVSTACK_GATE_NEUTRON=1 will enable neutron-networking
    DEVSTACK_GATE_TEMPEST_DISABLE_TENANT_ISOLATION=1 will force to run
    the tests in serial.
Add integration bridge for neutron

Neutron needs an integration bridge, and the bridge name of the integration
bridge, therefore exporting it as DEVSTACK_GATE_XS_INT_BR so that it could be
picked up by devstack-gate scripts.
Check out a custom revision of devstack
Use custom devstack-gate
Export flat network bridge name

Neutron is unable to recognise XenServer network name-labels as
networks, it needs the bridge name, therefore we need to query xapi for
the bridge name, and export it as DEVSTACK_GATE_XENAPI_FLATNET so it
could be picked up by devstack-gate scripts.