Added gateway service port value

[luismiguelsaez-steercrm]

Adding the ability to change the Apisix gateway's service pod target port. While working with cloud providers like AWS, the node ports are automatically configured based on the Kubernetes Service ports. When we want to offload the TLS to the load-balancer, we need to send the HTTP traffic sent to the Service's TLS port to the Pod HTTP port and, with the current configuration, we cannot do it because the same value ssl.containerPort is used for:

• Gateway service's tls port target port to the Pod
• Pod's TLS container port
• Internal Nginx configuration, defined in the ConfigMap