Rename Gerrit branch env to GERRIT_BRANCH in tests

Change-Id: I58227c8bef3885fece3631dfc8b41c3093dcfdf8

src/test/docker/gerrit/Dockerfile

diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index 74ba1a2..641028d 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -10,7 +10,7 @@
     gettext \
     && yum -y clean all
 
-ENV GERRIT_VERSION stable-3.11
+ENV GERRIT_BRANCH stable-3.11
 
 # Add gerrit user
 RUN adduser -p -m --uid 1000 gerrit --home-dir /home/gerrit