Horizon plugins needs horizon on master branch for our
master development and test.
Recent changes in CI infra and horizon force to change horizon
installation, and added tox_helper.sh as trivial fix.
Now, reasonable fix found for horizon plugins,
so this patch fixes them.
Also, use python3.5 for js jobs.
Change-Id: Ibcaf177db21f96e60ee2b222ddd24bc61c3ee9e4