Add support for multiple bundles

This change brings in support for multiple bundles to be downloaded
and activated OPA.

This is enabled by using the new config option `bundles` to define
the bundles, and deprecates the older `bundle` option.

The new `bundles` keyword and structure is propagated through to the
decision logs, status API, provenance, stored manifests, etc. Check
out the doc changes for all the updated structures.

That being said any existing configuration using `bundle` will *not*
see the new structure, everything is intended to be backwards
compatible (almost to a fault).

Fixes: #721

Signed-off-by: Patrick East <east.patrick@gmail.com>