tools : engine-config's manual should be updated regarding new command --merge

engine-config --help should display the newly added
merge action.

Change-Id: I62a510f4c3dde79630af96ffb4ca010301b3cd13
Bug-Url: https://bugzilla.redhat.com/1091942
Signed-off-by: Ravi Nori <rnori@redhat.com>
(cherry picked from commit 776a8acf1741516e3d4b02d05ff0b2a4d75cef7c)