[FFe] FFe for 20.04 snap transition

[Adam Collard]

Ubuntu Focal currently has MAAS 2.6.0.

This is a Feature Freeze Exception request for MAAS to allow the package to move to a snap transition. Learning from, and copying code from the LXD snap transition, along with support in `do-release-upgrade` the MAAS team plans to release future MAAS versions as a snap.

Existing users will have their < 20.04 .deb upgraded to a snap installation, using the correct track and architecture for the expected release.

The binary packages from this package will be consolidated down to a single one which will contain the deb to snap transition logic.

Data migration
==============

Existing data will be untouched, and the snap configured to use whatever db is currently in use.

Testing
=======

Tests will be performed in several scenarios that cover the risks, notably:

- snapd package not installed
- airgapped environments which e.g. cannot reach the Snap Store
- i386 architecture transition

For more information on the QA process MAAS follows, please see https://wiki.ubuntu.com/MAASUpdates .

[Alberto Donato (ack)]

WRT i386, as focal won't support the arch anymore, no maas update will be available for i386 users.