No ability to enable baremetal mechanism driver

[Jeff Hillman]

There is currently no configurable way to enable baremetal as a mechanism driver in the neutron-api charm.

in hooks/neutron_api_context.py the mechanism drivers are hardcoded based upon "Which drivers to enable are deduced from OpenStack release and charm configuration options"

```

def get_ml2_mechanism_drivers():
    """Build comma delimited list of mechanism drivers for use in Neutron
       ml2_conf.ini. Which drivers to enable are deduced from OpenStack
       release and charm configuration options.
    """
    mechanism_drivers = [
        'openvswitch',
    ]

    cmp_release = CompareOpenStackReleases(os_release('neutron-server'))
    if (cmp_release == 'kilo' or cmp_release >= 'mitaka'):
        mechanism_drivers.append('hyperv')

    if get_l2population():
        mechanism_drivers.append('l2population')

    if (config('enable-sriov') and cmp_release >= 'kilo'):
        mechanism_drivers.append('sriovnicswitch')
    return ','.join(mechanism_drivers)

```

There appears to be no detection of either ironic-api, neutron-api-plugin-ironic or a nove-compute deployed with virt-type ironic.

[Jeff Hillman (jhillman)]

I did a test once we got ironic working. i reverted to -290 of neutron (what I started with) and it does work and has connectivity. The port does still show down, and with a binding_failed, but the ironic node is functional.