workaround frequent cleanup problems on win7

see also https://github.com/reficio/p2-maven-plugin/issues/12

the idea was, that the user can specify
    -Dorg.reficio.clean.relyOnMaven=true
if the user prefers to skip cleanupEnvironment() completely and rather rely on
Maven to cleanup at beginning of next build ...
    mvn clean p2:site

Cheers!

p.s. unfortunately, I could not run tests, because my m2e rejects plugin execution for mycila, gmaven and plexus because they are not covered by default lifecycle configuration