Complete Rubocop compliancy

- Ensure all files are rubocop compliant following commits
- Move to use a single .rubocop.yml file
- Update StrainerFile to reflect the single .rubocop.yml

Change-Id: I6d7b30aae4d839a2c8c2ebebbf5e9b53518db71e
Addresses: blueprint rubocop-for-image