[Rename] Savanna to Sahara in the client

https://github.com/openstack/python-savannaclient (soon to be python-saharaclient)

all uses of Savanna need to become uses of Saraha

the sahara cli need not maintain a savanna alias for compatibility

the directory / module structure changes will require cross repo coordination

this is a large effort, its owner should feel free to add multiple work items and recruit assistance