mon: discover mon addrs, names during election state too

Currently we only detect new mon addrs and names during the probing phase.
For non-trivial clusters, this means we can get into a sticky spot when
we discover enough peers to form an quorum, but not all of them, and the
undiscovered ones are enough to break the mon ranks and prevent an
election.

One way to work around this is to continue addr and name discovery during
the election.  We should also consider making the ranks less sensitive to
the undefined addrs; that is a separate change.

Fixes: #4924
Backport: dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
Tested-by: Bernhard Glomm <bernhard.glomm@ecologic.eu>
(cherry picked from commit c24028570015cacf1d9e154ffad80bec06a61e7c)
ceph-post-file: use mktemp instead of tempfile

tempfile is a debian thing, apparently; mktemp is present everywhere.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit e60d4e09e9f11e3c34a05cd122341e06c7c889bb)