mdadm/test: enable clustermd testing under clustermd_tests/

mdadm/test: enable clustermd testing under clustermd_tests/

For clustermd testing, it needs user deploys the basic cluster
manually, test scripts don't cover auto-deploy cluster due to
different linux distributions have lots of difference.
Then complete the configuration in cluster_conf, please refer to
the detail comments in 'cluster_conf'.

1. 'func.sh' source file, it achieves feature functions for
   clustermd testing.
2. 'cluster_conf' configure file, it contains two parts as
   the input of testing.

Signed-off-by: Zhilong Liu <zlliu@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>