Bump sahara client to 0.7.3

Need new Sahara client to finish blueprint cluster-secgroups in
Horizon. E.g. https://review.openstack.org/#/c/118493/ is waiting 
for a new client.

Change-Id: If1c01a99c9f4448218c46c0898cd6b7b2d143f21
Blueprint: cluster-secgroups
Merge "Bump sahara client to 0.7.3"