Split sahara into sahara-api and sahara-engine

Split monolitic sahara-api into two services: sahara-api and
sahara-engine. The former is a user-facing interface, the later -
service doing all the work. Sahara-api sends tasks to sahara-engine
via oslo.messaging. See the blueprint for details.

Notes:

* Used the following Climate RPC code as a baseline:
  https://github.com/stackforge/climate/blob/master/climate/utils/service.py
  hence added Julien Danjou to licence header.
* Removed the old contents of sahara/utils/rpc.py - that is a prehistoric
  stuff not used anywhere.
* sahara-api still depends on eventlet, while we want to drop this dependency
  in the end.
* periodics run in sahara-api. They shold be moved to engine later.

Partially Implements: blueprint scalable-savanna
Change-Id: I64275a757b539f3fcddd6e993d6614d492745226
Merge "Split sahara into sahara-api and sahara-engine"