create a default value for nginx_enablerepo

defaults/main.yml

@@ -9,6 +9,9 @@ nginx_yum_repo_enabled: true
 nginx_ppa_use: false
 nginx_ppa_version: stable
 
+# Use an alternate repo on Centos/RHEL
+nginx_enablerepo: ""
+
 # The name of the nginx package to install.
 nginx_package_name: "nginx"