Cells API calls return 501 when cells disabled

When cells is not enabled the cells API calls should return
a 501 NotImplemented rather than have the internals
eventually raise various errors which may include timeout
errors.

+DocImpact: cells API calls will now return a 501 if cells is disabled
Closes-Bug: 1288214

Change-Id: I07e69bbe11ec88cd3f07b47846f3a0a5c9e6f3dc
Merge "Cells API calls return 501 when cells disabled"