Change database fields to NOT NULL

[Glutexo]

No host’s fields can ever be empty. Still they are declared as NULL. Let’s switch them to NOT NULL. This applies to:

• account – no host can live without an account
• display_name – defaults fo fqdn or id
• created_on – filled in with a current timestamp
• facts – defaults to an empty dictionary
• tags – remove this field entirely
• canonical facts – validated to be a dictionary with at least one item from a fixed set
• system_profile_facts – defaults to an empty dictionary

I don’t think that we need any sophisticated validation logic in the database layer. The fields are still there with their type and nullability is a very basic property of the field. Moreover, non-nullable fields are more performant – faster to index and search. Not being able to insert a record without a value is in my eyes equally important as not being able to insert a value of a wrong type.