restapi: Add ballooning to RSDL metadata

This patch adds the documentation of the ballooning parameter to the
RSDL metadata.

Change-Id: Idc73af8bc7b2dc74196f247f464dd31622b8797d
Bug-Url: https://bugzilla.redhat.com/1149188
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>