sheep: define DEFAULT_CLUSTER_DRIVER

This simplifies group.c a bit and corrects the help message about the
default cluster driver when we don't compile a corosync driver.  In
addition, this would help us to switch the default driver easily if we
would change the default in future.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>