Pin tempest version

Currently tempest will not install due to us pinning
python-saharaclient to 0.7.5 while tempest requires 0.7.6.  Rather than
bumping python-saharaclient, this change will pin tempest to the commit
prior to requirements.txt updating.  This type of change was already
done to tempest in stable/icehouse.

Change-Id: I12fad99d32d6bc8c7096af0ac8bed5a849c24da8
Closes-Bug: #1402713
(cherry picked from commit 4790cf46d645bd77137bf6f62172623de583e59c)
Merge "Pin tempest version" into stable/juno