Renaming all Savanna references to Sahara

WARNING:
----
Before merging this commit the alias for
http://sahara-files.mirantis.com needs to be in place.

Also before merging this commit the new openstack git project must be
available at https://git.openstack.org/openstack/sahara-image-elements/

NOTE:
----
The file 'elements/hadoop-hdp/source-repository-hadoopswift' contains a
link to the HortonWorks repository that holds the Hadoop Swift rpm, this
link needs to be updated when HortonWorks makes the change.

Implements: blueprint savanna-renaming-image-elements
Change-Id: Icb9a992f8545535af3a111580ce7c9622d754c67