JBIDE-15009 : various fixes to the offline script

- forces d/l of resources:2-5 plugin

- runs install instead of verify, as it's necessary for dependency
  resolution in certain builds (ex. building only an ear requires wars/ejbs
  to be available (i.e. installed) in the local repo.

  There's the special case of resteasySimple.zip, which has a bad jetty
  configuration and unskippable integration tests, causing the build to
  fail. Instead of install, the ultimate goal is downgraded to package.

- gatein examples were not picked up because their pom.xml is one folder
  down the zip structure. the script now checks one folder deep for
  pom.xml if none is found at the root

- Installed artifacts are cleaned up if possible, to avoid polluting the
  local repo