mgr: update metadata if an osd just joins

mgr: update metadata if an osd just joins

instead of using "front_address" for checking if we have a new OSD
reusing existing a known identify shows up in the osdmap, it'd be
simpler to compare the up_from epoch with the osdmap's epoch. as
objecter will subscribe **every** osdmap after mgr boots. so mgr
should be able to see the osdmap when the osd joins the cluster
where the up_from epoch is identical to osdmap's epoch.

this way is simpler than existing approach. but it will involve
more overhead if osd reboots frequently without changing their
metadata.

before this change, the metadata is requested/updated only if
the public (front) address is changed.

after this change, the metadata is requested/updated whenever
an osd reboots.

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