RAPI: Add new request data format for instance creation

As mentioned in commit d975f482, the current way of creating an
instance via RAPI is not very flexible. With this patch, a new
instance creation request data format is introduced and documented.
Support can be detected by checking the list of features returned
by the /2/features resource for the value "instance-create-reqv1".

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>