Make the archiving of output.xml optional

This is a first attempt to implement the request [JENKINS-21736]
Couple of notes on my work:
- I added this new option in the advanced section
- default behaviour of the plugin remains identical (by default, option not checked)
- I added a system test to check that when option is checked the output.xml files are not copied
- wording of labels/helps/methods can be reviewed of course