REGTESTS: fix maxconn update with agent-check

Correct the typo in the parameter used to update the 'maxconn' via
agent-check. The test is also completed to detect the update of maxconn
using CLI 'show stats'.

(cherry picked from commit e500e593a79e3797eada5a839aeee017b002d7bf)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed