Add a new federation API

Some of the Magnum supported COEs (for now just kubernetes) support federating multiple clusters.

Taking kubefed as an example, it is complex enough (cluster context setup, credential setup, ...) to justify adding it to the Magnum functionality. We can envision something like:
```
$ magnum cluster-create --cluster-template kubernetes --node-count 5 --name mycluster1
$ magnum cluster-create --cluster-template kubernetes --node-count 5 --name mycluster2

$ magnum cluster-fed-init --fed-name myfedtest --dns-zone 'cluster.myfedtest' mycluster1
$ magnum cluster-fed-join --fed-name myfedtest
```