Key prefix for secret variables is changed if envVarPrefix is unset in source instance #156
Description
If an application hasn't set .spec.gcp.sqlInstances[].databases[].envVarPrefix, one is autogenerated using the following convention:
NAIS_DATABASE_${INSTANCE}_${DB_NAME}_
As part of the migration process, this means that the prefix (and thus all the keys) will change as we're migrating to a new instance.
The easiest fix would likely to inform the users about this through the CLI outputs and/or the guides in the documentation, so that they update their application accordingly. Alternatively, we could automatically set envVarPrefix to match the previous instance name though this still requires the user to update their application manifest.