Disabling the assertion auto-import job on Core should be possible

Currently the assertion import job is always running on all Ubuntu Core 16 images. Due to the nature of the design it has to probe for disks/USB keys/etc which might be slow in replying (due to a slow controller, or because there is an additional (low speed) USB key with data for an application snap etc). This can slow down the boot process of a device.

There are many scenarios where consumer images will not use the assertion import feature at all (pre-configured at factory, pre-configured via cloud-init) or even consider it a security risk to allow access even through something signed …

The core snap should allow to disable this service through a services config option (similar to how we disable ssh or rsyslog today) that can be pre-seeded from a gadget snap configuration option.