Specification of mirror in jqassistant.yml should be enough in isolated environments

[SebastianWendorf]

Task Description

Currently it's necessary to specify a mirror URL twice in the jqassistant.yml when configuring a mirror for Maven Central in environments where direct access is closed off.
This should be simplified in a way that assumes Maven Central as a default.

Definition of Done for the Implementers

Remove this hint: these checkboxes can be checked like this: [x]

• We got a final feedback from the reporting user (if applicable)
• Unittests have been written (if applicable)
• Integration tests have been written (if applicable)
• Test coverage is the same or even better then before (if applicable)
• Documentation has been written (if applicable)
• Added a note on the new feature to the release notes (if applicable)