FIXED JENKINS-15331 by changing Util.deleteContentsRecursive, Util.deleteFile...

FIXED JENKINS-15331 by changing Util.deleteContentsRecursive, Util.deleteFile and Util.deleteRecursive so that they can retry failed deletions.
The number of deletion attempts and the time it waits between deletes are configurable via system properties (like hudson.Util.noSymlink etc).
Util.DELETION_MAX is set by -Dhudson.Util.deletionMax.  Default is 3 attempts.
Util.WAIT_BETWEEN_DELETION_RETRIES is set by -Dhudson.Util.deletionRetryWait.  Defaults is 100 milliseconds.
Util.GC_AFTER_FAILED_DELETE is set by -Dhudson.Util.performGCOnFailedDelete.  Default is false.

Added unit-tests for new functionality.