Add resource_data field for free-form JSON data.

This can be used to store arbitrary resource-specific data, such as
SSH keys.

Blueprint resource-data.

Change-Id: Ic148bd131f528a676904c8f37fd6deb936917152
Merge branch 'resource-data' into cloud-server
Move self._private_key.

Change-Id: I05ac87544838e69fd5744746a06b9a8086edfef7