Db schema configuration option

[tisnik]

Description

• Added new configuration option: (db)schema
• Updated documentation accordingly
• Updated deployment accordingly (now it's set to fixed value!)
• Configuration checking code
• Unit tests for the new code

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Unit tests (no changes in the code)
• Documentation update
• Configuration update

Testing steps

To be checked on CI.

Checklist

• make before_commit passes
• updated documentation wherever necessary
• added or modified tests if necessary
• updated schemas and validators in insights-data-schemas in case of input/output change