Add "--param WILDCARD_POLICY=Subdomain" in the 2.6 On-premises installation instructions

The installation instructions for Red Hat 3scale 2.6 On-premises are the same for both with and without a Wildcard Routing configured:
https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.6/html-single/installing_3scale/index#importing-the-threescale-template

Without Wildcard Routing:
~~~
$ oc new-app --file /opt/amp/templates/amp.yml --param WILDCARD_DOMAIN=<WILDCARD_DOMAIN>
~~~

With Wildcard Routing:
~~~
# oc new-app --file /opt/amp/templates/amp.yml --param WILDCARD_DOMAIN=<WILDCARD_DOMAIN>
~~~

On With Wildcard Routing, we need to add --param WILDCARD_POLICY=Subdomain as it's described on 2.5 On-premises:
https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.5/html-single/installing_3scale/index#deploying_3scale_on_openshift_using_a_template