Rename rabbit_virtualhost to rabbit_virtualhost

This commit makes kickstack work together properly with the heat module
again; recently, inside the heat module, rabbit_virtualhost was renamed
to rabbit_virtual_host.
Add ML2 settings; return proper value for neutron_tenant_network_type

This commit adds a number of parameters required for ML2 compatibility;
it also ensures that $neutron_tenant_network_type is set correctly. If
ML2 is in use, $neutron_tenant_network_type needs to be an array. If the
old OVS plugin is in use, it needs to be a string.
Add database configuration to neutron.conf

Previous versions of Kickstack would have the database configuration in
the plugin. By now, apparently, the recommended mechanism is to have the
DB configuration happen in neutron.conf -- at least the ML2 class of the
Neutron module doesn't even implement database parameters anymore. Thus,
we need the database configuration in neutron.conf. This commit adds the
required parameters to the neutron class.
Add ML2 to the list of allowed interface drivers and plugins

Previously, Kickstack supported the OVS plugin and the Linuxbridge
plugins only; by now, it supports the ML2 plugin as well. We need
to have ML2 as a choice because of that. This commit adds support
for ML2 to the DHCP and L3 agent configuration and to the config
class.
Add ML2 parameters to the kickstack::neutron::plugin class

This commit enables the kickstack::neutron::plugin class to support the
ML2 plugin in OpenStack Neutron. It ensures that network_vlan_ranges and
tunnel_id_ranges are set properly depending on the plug-in that is used.
For OVS or Linuxbridge setups, these need to be strings. In ML2 setups,
they need to be arrays.

It also deletes a workaround for a previous bug in the Neutron plugin
for Open vSwitch that avoided proper functioning of the firewall_driver
variable. The bug in the Neutron plugin has been fixed by now, and thus
the workaround is not necessary anymore.