common: pick_addresses: fix bug with observer class that triggered #5205

The Observer class we defined to observe conf changes and thus avoid
triggering #5205 (as fixed by eb86eebe1ba42f04b46f7c3e3419b83eb6fe7f9a),
was returning always the same const static array, which would lead us to
always populate the observer's list with an observer for 'public_addr'.

This would of course become a problem when trying to obtain the observer
for 'cluster_add' during md_config_t::set_val() -- thus triggering the
same assert as initially reported on #5205.

Backport: cuttlefish
Fixes: #5205

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 7ed6de9dd7aed59f3c5dd93e012cf080bcc36d8a)