Move code from _nm_utils_team_config_get to the brand new
_json_team_add_defaults function without any change.
Then remove the duplicated code from _nm_utils_team_config_equal and
leverage the new function. Here the only functional change is that
the defaults for "notify_peers" and "mcast_rejoin" for the
"activebackup" runner are added (the only case in which their default
values are different than 0).