NEXUS-4580: New task parameter for EmptyTrashTask

Ability to specify repository parameter for task.
NEXUS-4580: IT
NEXUS-4580: Field order changed

As Alin commented, to have it aligned with other
tasks having repo parameter.
NEXUS-4580: No code change, comments fixed
Merge pull request #127 from sonatype/nexus-4580-empty-trash-per-repo

NEXUS-4580: New task parameter for EmptyTrashTask