RAPI: Pass depends body arg (if any) to opcode

RAPI: Pass depends body arg (if any) to opcode

Most rlib2 classes override the default _GetDefaultData() method with
custom methods that parse a request's body and query args and return a
(body, specific_static) tuple eventually passed to FillOpCode().
Job dependencies are defined in the `depends` body argument that might
get lost because most of those methods return {} as the body. In order
not to modify every custom method we update the returned body with
the `depends` argument of the original request body for all RAPI calls
inside OpcodeResource._GenericHandler(). It's up to the client to
include the `depends` argument with the job dependencies if desired.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Hrvoje Ribicic <riba@google.com>