[7.0] Openshift - Replace hard-coded URLs in this document with attributes

There are still hard-coded URLs to other documentation defined in topics in this repository that should be moved to the `document-attributes.adoc` file so they can be more easily monitored for changes.

openshift-install-config.adoc:

• https://access.redhat.com/documentation/en-US/JBoss_Enterprise_Application_Platform/6.1/html-single/Security_Guide/index.html#Generate_a_SSL_Encryption_Key_and_Certificate[Generate a SSL Encryption Key and Certificate]

openshift-clustering.adoc:

• link:https://access.redhat.com/documentation/en-us/openshift_container_platform/3.5/html/architecture/core-concepts#labels[label set at the service level]

openshift-eap-sso.adoc:

• link:https://access.redhat.com/documentation/en/red-hat-xpaas/version-0/red-hat-xpaas-sso-image/#deploy_sso_xpaas_image_with_the_sso_enabled_jboss_eap_xpaas_image[Red Hat SSO for OpenShift Image]