Add ubuntu-desktop-session snap to the list

[sergio-costas]

It is required to be able to automagically install it for future versions.

[sergio-costas]

@jhenstridge When you have some spare time, can you review this, merge it, and ask IS to sign it, please? I'm replacing the custom-modified ubuntu-core-desktop-init installer with the upstream ubuntu-desktop-init one.

[sergio-costas]

I'm doing this in two stages: first adding the ubuntu-desktop-init snap to the list, and when everything is in place and migrated, remove the old ubuntu-core-desktop-init (it requires more PRs both here and in core-base-desktop, like canonical/core-base-desktop#61 ).

[jhenstridge]

I would think you'd be able to test this change without updating the model. Using the makefile from this repo, you could do:

make pc-dangerous.img EXTRA_SNAPS="ubuntu-desktop-init=candidate"

Or if you're testing with a custom build of the base snap, include that in EXTRA_SNAPS too (separated by a space). If everything works, we can add remove ubuntu-core-desktop-init completely and only get IS to sign one new copy of the model.

[sergio-costas]

I already tested it, and works fine. That's why I want to add a PR for this.

[sergio-costas]

I even tested if signing the .model files with my own key, and worked too.