Update bosh-director routing

Previously, bin/bosh-director would have configured each API endpoint to be
a separate piece of Rack middleware. This resulted in poor performance as
every single API controller would have been tripped for every request.

This change restructures the routes such that each controller is mapped
directly to an endpoint instead, which should reduce the per-request overhead.

This change also includes a (necessary) change to the delete task route:

DELETE /task/:id -> DELETE /tasks/:id

This is because TasksController is mounted as /tasks, and /tasks/task/:id
doesn't make any sense.

Additionally, ErrandsController has been removed: these routes were nested
under deployments, so the routes and tests have been moved accordingly.

Signed-off-by: Michael Maximilien <maxim@us.ibm.com>