Provide a more generic run-tox.sh.

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
Merge "Provide a more generic run-tox.sh."