Document job-group defaults order of priority

Document job-group defaults order of priority

We discovered that job-group variable definitions appear to
have higher precedence than project definitions. This caused a problem
for us in global-jjb as we would like to allow projects to override
default variables.

We should make it best practice that global-jjb stops providing
job-group definitions and deprecating any existing job groups.

Via patch I2b824e3cc89667698db8ebfdd3ce43f551e961d7 we were able to
resolve this for JJB version.

Change-Id: I93c8256193933a04e8732963216f69424cef3a5f
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>