Change the enduser facing apects of the shell

Change all references to the project name in the help text to
'sahara', including any command options. Also change environment
variable refereces to 'SAHARA_XXX'.  Modify the corresponding tests.

Partial-implements: blueprint savanna-renaming-client
Change-Id: If29996a51c9e8d1c26b5e7050ffee58b135b829e