[RM-12374] Pass mon host list from ceph.conf to mon_create

[RM-12374] Pass mon host list from ceph.conf to mon_create

When using mon_create_initial(), we already have list of initial mon
members from the config file.  Pass it to mon_create() in args.mon
so that mon_create() does not throw an exception when it finds that
args.mon does not exist.

This is actually an optimization as well. mon_create_initial() always
looked up mon_initial_members from the config file, but didn't pass that
to mon_create().  mon_create() would then see that args.mon was empty
and then look up mon_initial_members *again*.  Now we only need to open
and read the file once.

Fixes: #12374
Signed-off-by: Travis Rhoden <trhoden@redhat.com>