Converts engine/clients/rest.py module into http package (as client.py)

* BaseRestClient is now BaseHTTPClient
* RestClient is now HTTPClient
* AutoMarshallingRestClient is now AutomarshallingHTTPClient
* Imports all moved and renamed items back into rest.py with their old
  names.
* Adds loud deprecation warning in old rest.py placeholder module.

Change-Id: I7c246b73fcc88e49383e0fc359aa720c74ce5272
Merge "Converts engine/clients/rest.py module into http package (as client.py)"