This relation is very similar to the ceph-client relation so add
it to this repo so they can share code. This will retire the
seperate repo *1 and also require an update to the central
index of interfaces to point at the new location.
*1 https://github.com/openstack/charm-interface-ceph-mds
*2 https://juju.github.io/layer-index/
Change-Id: I9f438bb678da1b69d8161390aad2cf58907bc1b5