[r=rogpeppe] environs/cloudinit: use StateServingInfo
We use StateServingInfo in MachineConfig, because it exactly
represents what we want to store there. We also change
the arguments to agent.NewStateMachineConfig to separate
out the StateServingInfo, allowing us to drop the extra params
struct type.
We also rename MachineConfig.StateServer to MachineConfig.Bootstrap,
because extra state server machines should not have StateServingInfo
in them - Bootstrap more accurately reflects its meaning.
https://codereview.appspot.com/83530043/