Bump python-saharaclient version

python-saharaclient>=0.7.5 is required by tempest, so we must make that
version available. AFAIK we're not actually using it for anything, so
increasing the version shouldn't cause a problem.

Related: #597
(cherry picked from commit 18bc25166360568b54744d8b1002914d75d1cca6)
Merge pull request #599 from hughsaunders/597-bp-proposed-juno

Bump python-saharaclient version