[Rename] Savanna to Sahara in the guestagent

https://github.com/stackforge/savanna-guestagent (soon to be sahara-guestagent)

all uses of Savanna need to become uses of Saraha

preliminary set of work items -
* setup.cfg
* docs
* oslo-incubator
* messaging topic (recommend not putting sahara in the new name)

this agent has never been released so backward compatibility is not a concern

corresponding code in main repo should be probably updated too

owner should feel free to add multiple work items and recruit assistance