restapi: Support generic types for "add"

Currently the RSDL builder doesn't support generic types as parameters
to the "add" operation. As a result the generation of the Java SDK
doesn't work correctly.

Change-Id: I60d4b44c183eaaa4a42737382bcc6ad33a5ff989
Bug-Url: https://bugzilla.redhat.com/1115610
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>