Make the ganglia graphs optional

[ericfranz]

We filter the clusters out that don't have ganglia config here: https://github.com/AweSim-OSC/osc-systemstatus/blob/fd1ccc5b1f7decad1eaa90fe89ace3fe980ca3e0/app.rb#L9-L14

Lets instead just hide the links to these clusters in the navigation bar if the ganglia configs are not available. Also, the panels on the main page should not be clickable if there are no graphs.