New app, Riak Admin API, evolution of Riak Control

[hmmr]

This PR introduces riak_admin_api, a new app that exposes a well-defined API over HTTP to enable external control and management of riak clusters.

The difference from the previous attempt (OpenRiak/riak_kv#125) is in that:

• the API is formally defined, with the intention for it to be usable from scripts as well as from a reference Riak Control app;
• entities meant to use this API (users, groups, permissions) are separate from those managed by the standard riak security subsystem;
• a set of CLI commands is implemented to assist in the initial setup (creation of the first user).

Related PRs: OpenRiak/riak_kv#152, OpenRiak/riak_test#28, OpenRiak/riak_core#47.

[martinsumner]

If the permissions check depends on action - then action should be in the request line (e.g. URL, method, query parameter), and not the body. That way a) the flow of the functions in the riak_api_web_handler behaviour is as expected, but b) anyone who wishes to use HTTP infrastructure to add controls (e.g. Web Application Firewalls) can also apply appropriate rules via standard configuration.

This would align with the KV API - where the activity (and the Bucket) can be determined from the URI so controls can be applied via standard proxy filter rules.

[hmmr]

If the permissions check depends on action - then action should be in the request line (e.g. URL, method, query parameter), and not the body.

I agree. I'll have it as a query parameter.