Allow add-ons to specify enabled in manifest. (#2150)
Allow add-ons to specify enabled in manifest. (#2150)

This restores previous functionality, such that we can pre-install
and enable certain add-ons in our images.

These are filtered at the addon-list level, such that we only allow
add-ons owned by "Mozilla IoT" to be enabled by default.
mozilla-iot/gateway,moziot/gateway,mozilla-iot/gateway,mozilla-iot/gateway,moziot/gateway,mozilla-iot/gateway,mozilla-iot/gateway,moziot/gateway