DEVOPS-2222 switch to dash instead of . so SSL works.

We have a wildcard cert for *.sandbox.edx.org but not for
*.*.sandbox.edx.org

playbooks/roles/launch_ec2/tasks/main.yml

@@ -86,7 +86,7 @@
     zone: "{{ dns_zone }}"
     type: CNAME
     ttl: 300
-    record: "preview.{{ dns_name }}.{{ dns_zone }}"
+    record: "preview-{{ dns_name }}.{{ dns_zone }}"
     value: "{{ item.public_dns_name }}"
   with_items: ec2.instances