Stop all daemons on cluster initialization

Addresses issue 145. When daemons are still running from a previous
cluster instance, starting the daemons again will fail.
“ConfigWriter.IsCluster” only looks at the configuration file, which
could already have been deleted.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>