Revert "Update yum source url to the new artifactory location"

This reverts commit 025aa70.

Author: joshcooper

tasks/install_shell.sh

@@ -117,7 +117,7 @@ if [ -n "$PT_yum_source" ]; then
   yum_source=$PT_yum_source
 else
   if [ "$nightly" = true ]; then
-    yum_source='https://artifactory.delivery.puppetlabs.net:443/artifactory/internal_nightly__local/yum'
+    yum_source='http://nightlies.puppet.com/yum'
   else
     yum_source='http://yum.puppet.com'
   fi