[r=axwalk],[bug=1270043] Eliminate MachineConfig client API
The MachineConfig API has been superseded by
ProvisioningScript for manual provisioning.
Manual provisioning has been updated to use
ProvisioningScript, and MachineConfig removed
as it was added in the 1.17.0 release, and so
is unsupported.
Accordingly, statecmd.MachineConfig has been
combined with statecmd.FinishMachineConfig,
and it deals directly with the environs/config
MachineConfig struct.
Fixes #1270043
https://codereview.appspot.com/52900045/