restapi: RSDL generation at build time

As of this patch, compiling rest-api with maven will generate
rsdl.xml, rsdl_gluster.xml files, store them in: .../target/classes,
and package them into restapi-definition.jar.

This will make rsdl available without a running application server,
which makes for much more convenient sdk/cli project generation.
At runtime, instead of generating the RSDL, the application will
load the saved rsdl file and use it.

Change-Id: Idc13fb2feab8d065f38cecbda7da568463c30453
Signed-off-by: Ori Liel <oliel@redhat.com>