Disable per-user rate limiting by default

The rate limiting enabled via api_rate_limit, is per user rate limiting
which does not protect against intentional DOS, instead DOS rate
limiting should be elsewhere. Furthermore useful values for this are very deployment
specific.

* Private deployments simply don't need it.
* Public deployments need something more sophisticated (queue depth based,
  not static values).
* Folks that do want it as belt-and-braces want semi-decent defaults
  but there is no need to default it on.

Part of more sane defaults effort.

DocImpact

Change-Id: I5451f6f90a6448c838612f180f0af750213a62bf
Merge "Disable per-user rate limiting by default"