Add an explicit BRANCH_NAME to tox_install.sh

tools/tox_install.sh exists to deal with installing neutron without
neutron existing as a pypi package.

The way this operates causes problems in the periodic jobs on the stable
branches.  Make it explicit which branch to install in the gate
(zuul-cloner) and via pip.

This is done with a BRANCH_NAME variable to make it trivial to change
once "master" is tagged as stable.

Change-Id: I6a5b63fa82a6a7480adadd2f65463619fd077f7f

tools/tox_install.sh

@@ -15,6 +15,7 @@
 
 ZUUL_CLONER=/usr/zuul-env/bin/zuul-cloner
 neutron_installed=$(echo "import neutron" | python 2>/dev/null ; echo $?)
+BRANCH_NAME=master
 
 set -e
 
@@ -29,12 +30,12 @@ if [ $neutron_installed -eq 0 ]; then
     echo "ALREADY INSTALLED" > /tmp/tox_install.txt
     echo "Neutron already installed; using existing package"
 elif [ -x "$ZUUL_CLONER" ]; then
-    export ZUUL_BRANCH=${ZUUL_BRANCH-$BRANCH}
     echo "ZUUL CLONER" > /tmp/tox_install.txt
     cwd=$(/bin/pwd)
     cd /tmp
     $ZUUL_CLONER --cache-dir \
         /opt/git \
+        --branch $BRANCH_NAME \
         git://git.openstack.org \
         openstack/neutron
     cd openstack/neutron
@@ -42,7 +43,7 @@ elif [ -x "$ZUUL_CLONER" ]; then
     cd "$cwd"
 else
     echo "PIP HARDCODE" > /tmp/tox_install.txt
-    $install_cmd -U -egit+https://git.openstack.org/openstack/neutron#egg=neutron
+    $install_cmd -U -egit+https://git.openstack.org/openstack/neutron@$BRANCH_NAME#egg=neutron
 fi
 
 $install_cmd -U $*