Study how Upsun environment variables are created and used, see if they match

[rfay]

Preliminary checklist

• I am using the latest stable version of DDEV
• I am using the latest stable version of this add-on

Expected Behavior

• Upsun creates environment variables for use by settings, etc, for example MARIADB_USER.
• I think they are named after the relationship name, which is free-form
• Upsun flex no longer has templates, so we can't really expect these things to always be of the same pattern
• The user's upsun configuration may use relationship names (and environment variables?) that don't match our expectations.