Simplify json types on tasks

Tasks seems to be using nlohmann::json in several places where they can
use simpler structures.  Change to using array_t where appropriate.
Also parse the json string at startup, given that that's the only place
we use it.

Change-Id: I864a48c135691289a61f1d0af9a65bfeaabe3cde
Signed-off-by: Ed Tanous <etanous@nvidia.com>