Add abstracted function about get monitor initial members.

Add abstracted function about get monitor initial members.

    To get monitor members feature will be reuse serval
    times in mon.py and osd.py. Due to this reason, I
    rewrite the helper function for this feature.

    1. Add get_mon_initial_members that can handle
       monitor member featching and configuration
       input.
    2. Let all works about monitor featching of
       mon.py and osd.py to use this abstracted function.

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

Signed-off-by: Vicente Cheng <freeze.bilsted@gmail.com>