Remove warning for existing output directory.
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 <62cccc0004df0a8e3c343b26b69e434f6aa9711c@redhat.com>
Reviewed-by: Dan Prince <62cccc0004df0a8e3c343b26b69e434f6aa9711c@redhat.com>
Tested-by: Jenkins

openstack-infra/reviewday,openstack-infra/reviewday,dprince/reviewday