MINOR: tcpcheck: Support 2-steps args resolution in defaults sections

MINOR: tcpcheck: Support 2-steps args resolution in defaults sections

With the commit eaba25dd9 ("BUG/MINOR: tcpcheck: Don't use arg list for
default proxies during parsing"), we restricted the use of sample fetch in
tcpcheck rules defined in a defaults section to those depending on explicit
arguments only. This means a tcpcheck rules defined in a defaults section
cannot rely on argument unresolved during the configuration parsing.

Thanks to recent changes, it is now possible again.

This patch is mandatory to support TCP/HTTP rules in defaults sections.