support secret="secret" in AJP nodes

The CVE-2020-1938 "mitigation" forces the use of a secret between httpd and the back-end.
<Connector port = "8009"
protocol = "AJP / 1.3"
redirectPort = "8443"
address = "YOUR_TOMCAT_IP_ADDRESS"
requiredSecret = "YOUR_TOMCAT_AJP_SECRET" />
Actually secret="secret" is support in mod_proxy_ajp but not in mod_cluster.
That prevents use using the mitigation.