Add forceLink() which replaces a link if it already exists.

Reduce the number of fileUtils.safeUnlink() callers.
safeUnlink() will be removed, using rmFiles() instead.

Change-Id: Idea4ab66cc2954fdb11606daa4a73ad3edb0c7ef
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11423
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>