Improve tools/tox_install.sh
Improve tools/tox_install.sh

Inspired from the tox_install script in the python-openstackclient, this
commit brings in the following improvements to glance_store'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: I1dfa42eac22f3f9515d69146a25c4f96e55523e4

openstack/glance_store,openstack/glance_store