Add new deps for GCE metadata.
Add ability to add instances via cloudprovider.

Adds the interface and makes it not implemented in all components except
GCE. Implements adding instances in GCE.
Adding provisioner service.

Service initially only exposes one endpoint:

/instances

Sending a POST to this endpoint requests the creation of a set of
instances.
Merge pull request #5 from vmarmol/provisioner

Provisioner