Inspired from the tox_install script in the python-openstackclient, this
commit brings in the following improvements to python-glanceclient's
tools/tox_install.sh:
* 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: Ibed7e2fbe35d2f2a520383e36ad31a5e7c8ef548