Up cron to 15 minute runs

5 minute runs are actually pretty close to overlapping due to polling
all the remote blogs.  I'm sure we can wait a few more minutes for a
blog post here.

Change-Id: I2a5455f0daadca79327e8a325f29c39a9dfde70d

manifests/site.pp

@@ -20,7 +20,7 @@ define planet::site(
 
   cron { "update_planet_${name}":
     command => "date >> /var/log/planet/${name}.log && cd /var/lib/planet/${name} && git pull -q --ff-only && planet /var/lib/planet/${name}/planet.ini >> /var/log/planet/${name}.log 2>&1",
-    minute  => '*/5',
+    minute  => '*/15',
     user    => 'root',
   }
 }