Integrate Magnum with Manila

Use cases where container applications require a shared filesystem:
- configuration files
- log file storage
- any other output from applications

Currently we only support Cinder, which has limited (or non working) multi-attach support and is not necessarily suited for these use cases. The gophercloud integration seems to be there:
https://github.com/gophercloud/gophercloud/tree/master/openstack/sharedfilesystems/v2

Integration with Manila could be done in a similar way to the Cinder one, by:
- setting the swarm/kub/mesos configuration as required to have an external volume driver
- make sure the volume drivers (rexray?) support handling of manila shares (for swarm + mesos)
- make sure the kubernetes openstack cloud provider supports handling of manila shares

Ideally we should support multiple protocols, but NFS would be a good start.

Handling of acls for access to the shares seems to also be protocol-dependent:
- nfs: list of ip addresses (floating ips? full subnet range of the cluster when using flat networks?)
- cephfs: ceph id accessing the cluster, and fetching the ceph keyring + setting the local ceph conf to pick it up