BUG/MINOR: config: allow to check HAPROXY_LOCALPEER in config

BUG/MINOR: config: allow to check HAPROXY_LOCALPEER in config

This fixes the commit d6ccd1738bae
("MINOR: startup: set HAPROXY_LOCALPEER only once"). HAPROXY_LOCALPEER could
be checked in the configuration to set some servers settings or listeners. So,
we need to set it just before we read the configuration at the second time.

Let's mark HAPROXY_LOCALPEER as "usable" in the configuration in the related
documentation chapter.

This should not be backported, as related to the latest master-worker
refactoring.