[Technical debt] Make self-managed SCIM code adhere to rubocop rules

The following discussion from !180831 (merged) should be addressed:

• @paulobarros started a discussion:

  note: The current instance SCIM code was created before this cop, and if we were to comply with it, part of the functionality would live in a different place than the current code. IMHO it's better to make this refactoring separately and at once.