Support trusted clusters

[jaxxstorm]

https://github.com/gravitational/teleport/blob/master/docs/admin-guide.md#trusted-clusters

[jaxxstorm]

Required params:

cluster_name:
trusted_clusters:
   - key_file:
`tunnel_addr:`

[jaxxstorm]

Whoops closed the wrong issue

[jaxxstorm]

So essentially, what we end up with is this:

trusted clusters can be an array, with keys:

key_file:
allow_logins:
tunnel_addr:

We need to iterate over this array of hashes