Replaced all Savanna words in class names

* All *Savanna* are replaced with Sahara
* Affected are:
  - SavannaBase and _SavannaBase
  - SavannaException
  - SavannaTestCase
  - SavannaWithDbTestCase
  - SavannaPeriodicTask
  - SavannaImage and SavannaImageManager
* Didn't affeceted is SavannaKeyPair since it's being removed in
  https://review.openstack.org/#/c/79442/

Change-Id: I06883a5db7ad5389914cc3eb7e61c4dc142119cf
Partially implements: blueprint savanna-renaming-service
Replaced or removed Savanna words in comments

Partially implements: blueprint savanna-renaming-service

Change-Id: I36914f7648a12c470a20509fb53467c40b483b7c
Merge "Replaced all Savanna words in class names"
Merge "Replaced or removed Savanna words in comments"