box: make box_check_uri_set return parsed uri set

To handle the case when box.cfg.replication isn't changed (required for
SSL certificate update), we need the uri set in box_set_replication.
We could obtain it with cfg_get_uri_set, but calling this function right
after box_check_replication, which already creates the uri set looks
ugly. Let's make box_check_uri_set, box_check_replication, and
box_check_listen return the uri_set, like box_check_uri does.

While we are at it, fix a memory leak in box_check_config, where we
forgot to destroy the uri returned by box_check_bootstrap_leader.
The leak is insignificant and may only happen once so this doesn't
deserve to be mentioned in the changelog.

Needed for tarantool/tarantool-ee#432

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring