[r=dimitern] api/provisioner: Allow adding networks and NICs
This adds two new Provisioner API calls:
* AddNetwork (exposed as st.AddNetworks bulk call on
the client-side provisioner API)
* AddNetworkInterface (exposed in the client-side API
as machine.AddNetworkInterfaces bulk call)
These will be used to set the networks/interfaces that
will be configured by MAAS at provisioning time.
Next, we'll make the necessary changes to StartInstance()
in general (and in provider/maas specifically) to return
what networks will the machine start with.
https://codereview.appspot.com/84570043/
R=gz