Add indirection (REST usable) to manipulate instrumentation

This adds two REST API:

• /instrumentation_listener(_search)/<instrumentation_name>

GET /instrumentation_listener/<instrumentation_name>
Gives details about a given instrumentation listener

GET /instrumentation_listener_search/unused
Lists all instrumentation listeners loaded in the system

PUT /instrumentation_listener/name
Allows to enable or disable a given instrumentation listener (supports
only pson). The body should be {"name":"instrumentation_name","pattern":<pattern>,"enabled":true}

DELETE /instrumentation_listener/<instrumentation_name>
This fully unsubscribe a listener

• /instrumentation_data/<instrumentation_name>

Only GET is supported. This returns the accumulated data of the given
instrumentation name if the listener supports data aggregation.
This is the only way so far to get access to the instrumentation data.
The data format is specific to the given listener.

Note: actual default auth.conf doesn't allow to interact with these
two indirections.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>