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: I484637d2502f276313edc2279078a200ebc2689a