Redesigned the HTTP<->API mapping.

Comments would be appreciated. @okrieg, @pjd, @henn
For connect/detach, I recommend pulling the hnic label into the URL and just keeping the network in the body in order to maintain the unvarying portion outside of the body.

Also, we could shorten "connect_network" to just "connect" since we'll have the context.
 
Thus, the new URLs might look like
    POST /headnode/\<hn_label\>/hnic/\<hnic_label\>/connect {network=\<network_label\>}
    POST /headnode/\<hn_label\>/hnic/\<hnic_label\>/detach {network=\<network_label\>}
or perhaps detaching the network could look like:
    DELETE /headnode/\<hn_label\>/hnic/\<hnic_label\>/\<network_label\>