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.
Change-Id: Ieae9bfdfed113795c0a7065964781914a2d041ce
Depends-On: I506e9c761ad1af3126db2c4778aa9dbbcf340130