Add new unit tests for get_mon_initial_members().
Add new unit tests for get_mon_initial_members().

    Add the unit tests for new abstracted function.
    Detail as below:

    1. Test Mon value is None and
       error_on_empty is True.
    2. Test Mon value is None and
       error_on_empty is False.
    3. Test Mon value is single item.
    4. Test Mon value is multiple item.

If you have any questions, feel free to let me know.
Thanks!

Signed-off-by: Vicente Cheng <0e6eee66936b7a4b2225fd2d5ebb0c25872c5879@gmail.com>

ddiss/ceph-deploy,ddiss/ceph-deploy,SUSE/ceph-deploy-to-be-deleted,osynge/ceph-deploy,alfredodeza/ceph-deploy,ceph/ceph-deploy,jumpstarter-io/ceph-deploy,Vicente-Cheng/ceph-deploy,isyippee/ceph-deploy,osynge/ceph-deploy,zhouyuan/ceph-deploy,imzhulei/ceph-deploy,shenhequnying/ceph-deploy,rtulke/ceph-deploy,SUSE/ceph-deploy-to-be-deleted,alfredodeza/ceph-deploy,rtulke/ceph-deploy,branto1/ceph-deploy,zhouyuan/ceph-deploy,SUSE/ceph-deploy,branto1/ceph-deploy,Vicente-Cheng/ceph-deploy,ceph/ceph-deploy,codenrhoden/ceph-deploy,ghxandsky/ceph-deploy,shenhequnying/ceph-deploy,trhoden/ceph-deploy,SUSE/ceph-deploy,codenrhoden/ceph-deploy,imzhulei/ceph-deploy,ghxandsky/ceph-deploy,trhoden/ceph-deploy,jumpstarter-io/ceph-deploy,isyippee/ceph-deploy