Savanna -> sahara rename

Change-Id: I69fe6d73a7a9218cfc9ae6cef0ae5f7dd04f9902
Update Sahara (ex. Savanna) glossary term

It based on approved OpenStack Data Processing program mission:

https://github.com/openstack/governance/blob/master/reference/programs.yaml

Change-Id: Ic0f0d20de5127d7d1e79afdb9e7277b66c8ac7a9