Renaming Savanna to Sahara

* File fuel_health/savanna.py was renamed to fuel_health/sahara.py

* File fuel_health/tests/platform_tests/test_platform_savanna.py was
  renamed to fuel_health/tests/platform_tests/test_platform_sahara.py

* File fuel_health/tests/sanity/test_sanity_savanna.py was
  renamed to fuel_health/tests/sanity/test_sanity_sahara.py

* Variables, methods, classes, messages were changed too

Change-Id: I26b797206e4fa61fb41dfbcf2903c7dc836cfcb8
Merge "Renaming Savanna to Sahara"