MINOR: check: allocate default check ruleset for every backends

MINOR: check: allocate default check ruleset for every backends

Allocate default tcp ruleset for every backend without explicit rules
defined, even if no server in the backend use check. This change is
required to implement checks for dynamic servers.

This allocation is done on check_config_validity. It must absolutely be
called before check_proxy_tcpcheck (called via post proxy check) which
allocate the implicit tcp connect rule.