qa/tasks/rebuild_mondb: use monmap to properly name the mons

qa/tasks/rebuild_mondb: use monmap to properly name the mons

We used to rely on the monmap bootstrap code to magically create a valid
monmap with named mons because our old-style ceph.conf had mon_addr
values in each mon.foo section.  Instead, just feed it a real monmap
from pre-destruction.

In practice, a user can manually generate this monmap, or rename the
mons after the fact with --inject-monmap, or whatever.  Out of scope
for this test, so we just do the simplest thing to make the rebuild test
work.

Signed-off-by: Sage Weil <sage@redhat.com>