Restore backwards compatibility for image register

Restore backwards compatibility for image register

Refs: Bug #4276

New format:

/image register Name container:path/to/image

Old format:

/image register Name pithos://user-uuid/container/path/to/image

Now, the old format is also recognized. It is planed to be depricated in a
future version, though

Also, uniformize guides and doc responses for image register