Rename run-tox.sh to run-unittests.sh.
Add a new run-tox.sh that takes the tox environment name as its first
argument instead of the python version number suffix. This makes it
easier to run alternative jobs through tox (building docs, building
with different versions of dependencies, etc.).
Based on feedback in https://review.openstack.org/#/c/38218/
Change-Id: I7e5a23fbe88af94bbfbbed4a47c5eb6527ab1d47