Fix KeyError in promote endpoint

[nadzyah]

This PR closes RTW-132

Here we use "ubuntu" as default snap store unless otherwise specified in artefact source. Also, I've added the test to make sure it works

Also, I've added the following migrations for the Artefact model:

• to make sure that existing snap artefacts have "store" key in the source field. If not, we set it to be "ubuntu"
• to require the "store" key in source field for snap artefacts when they are created or updated. If the field is not specified, PostgreSQL raises an exception

Also, I've modified the tests and seed_data script to specify the "store" key in sample data.

[nadzyah]

While working on this fix and creating an empty migration, I noticed that there are other changes that didn't have any migration. So, one migration was created automatically by alembic

[omar-selo]

Looks good. Mainly an issue that I caused before with unique index

[jocave]

Don't feel I can sign-off the implementation, but the description of the changes sounds fine.

My only question is whether we should validate the value provided for store?

[nadzyah]

Don't feel I can sign-off the implementation, but the description of the changes sounds fine.

My only question is whether we should validate the value provided for store?

I’ve added the value validation locally. I’ll push them after fixing all the tests (or finally finding the reason why they fail)

[omar-selo]

Good job. Overall I think this approach of validation is better than the one baked in the DB. Just a few small comments, nothing major

[omar-selo]

LGTM!