Update multi-tenancy documentation
Update multi-tenancy documentation

This updates the multi-tenancy documentation. The changes include:
- use the service name (e.g. the Bare Metal service) instead of the
  project name (e.g. ironic)
- fix some grammatical issues
- refer to the ironic conductor as 'ironic-conductor' instead of 'conductor'
- remove note about setting enabled_network_interfaces config option in
  ironic-api's config file; that is no longer needed since the checks
  have been moved to the conductor side (when node creation was moved).

Change-Id: Ic4b1512672c16c1bff5b43595abd90190e23b0fe

pshchelo/ironic,openstack/ironic,SauloAislan/ironic,NaohiroTamura/ironic,NaohiroTamura/ironic,openstack/ironic,pshchelo/ironic,SauloAislan/ironic