Adding python-saharaclient to requirements

The python-saharaclient is necessary for the Sahara
dashboard to interact with the Sahara-api service, so it is being
added to requirements.txt.

Change-Id: Ia3374a6792a4b0d2388f3826e88d92136a2b5e63
Partial-Implements:  blueprint merge-sahara-dashboard
Merge "Adding python-saharaclient to requirements"