merge GIT_BRANCH and GERRIT_BRANCH into BRANCH

merge GIT_BRANCH and GERRIT_BRANCH into BRANCH

GIT_BRANCH parameter is different; master or stable/danube in daily
jobs, but 'refs/changes/57/27657/6' in verify jobs. This breaks some
job builders triggered by the both types of jobs. We have GERRIT_BRANCH
parameter for verify jobs to identify stream and expected branch into
the patch will be merged after it got +2 and submitted.

To avoid further confusion and to have common job builders for daily
and verify jobs, this patch introduce BRANCH parameter.

GERRIT_BRANCH is now deprecated.

Change-Id: Ibcd42c1cd8a0be0f330878b21d3011f1ec97043b
Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>