cyclades: Add Image DB model

Create a new model, named 'Image', to store basic information about
images that have been used to create servers or volumes. The images are
stored in Cyclades (in addition to Pithos where they exist) in order to
preserve information about images that may be updated or deleted from
Pithos.
cyclades: Use Image model to generate image stats

Use the newly introduced Image model in order to generate statistics
about the images that are used for servers.
cyclades: Store image info for new servers

Store the information about the image that has been used to create a
server. Also, update image mocks to provide all the required fields.