OpenStack - Ensure inventory is collected for all tenants.

@Fryguy @blomquisg please review.

For OpenStack refresh, ensure inventory is collected for all of the
tenants the EMS can access.

Extended OpenstackHandle to include tenant iterators.
Updated EmsOpenstack and the openstack parser to use the OpenstackHandle accordingly.
Updated spec tests and rerecorded vcr cassettes.
It only calls that from the floating_ips_nova method. It calls @network_service.floating_ips from the floating_ips_neutron method. So, it won't break when the network service is neutron, but it may not get the ips for all the tenants.

The Havana environment does have neutron configured, but there's only one tenant in that environment, so I couldn't verify if floating_ips is tenant specific.