ceph-monstore-tool: rename mon-ids in initial monmap

ceph-monstore-tool: rename mon-ids in initial monmap

when ceph-mon starts, it checks to see if it's listed in the monmap, if
not it complains
```
no public_addr or public_network specified, and mon.a not present in
monmap or ceph.conf.
```
then bails out. normally, the monitor will try to rename its name in
monmap when performing "mkfs", but in our case, we are merely using the
"mkfs" monmap for passing the monmap built by ceph-monstore-tools, and
we don't actually go through the "mkfs" process. so, ceph-mon won't
rename when booting up.

in this change, user is allowed to specify the mon-ids in command line
when rebuilding mondb, the default mon-ids would be a,b,c,... if not
specified.

Signed-off-by: Kefu Chai <kchai@redhat.com>