Remove warning for existing output directory.

In our configuration puppet will manage the output directory, so it
is expected behavior for it to exist, removing warning. Also
switching to distutils.dir_util copy_tree since that allows for
copying of required supporting files into an existing output
directory.

Change-Id: I38b2c6ec47fd61814554a4b5007a83553b05aeb2
Reviewed-on: https://review.openstack.org/20647
Approved: Dan Prince <dprince@redhat.com>
Reviewed-by: Dan Prince <dprince@redhat.com>
Tested-by: Jenkins