Sync tools/tox_install.sh with python-openstackclient. This brings in
the following improvements:
* Do not leave temporary directory around, instead delete temporary
directory directly after usage (change
I939eae82dba3287fd4e4086128ebf4609a0e0770).
* Do not set ZUUL_BRANCH explicitely and remove unused if condition
(change I0077c986a17d6bb92791474e03d1e77776e9382f).
Change-Id: Ifb9c9da59c9c7d94362bbc82a389ebce518f59f1