Link to blackbox exporter on index.html

[sdarwin]

Add blackbox-exporter to the main page of demo-site.
Probably most users would be glad to have this, if installing prometheus on their own servers. Otherwise, easily deleted.

Another issue, index.html could benefit from being templatized, by including new variables:

• The domain name. Defaults to "demo.do.prometheus.io". Likely to change for each install.
• The link text. Currently says "Open demo", as default. Most other installations might say "Open console" instead because it won't be a demo.
• Potentially, a list of all consoles to display, starting with [grafana,prometheus,alertmanager,node,blackbox]

[sdarwin]

fyi - around 8 days ago a molecule dependency ansible-compat was upgraded from 0.5.0 to 1.0.0, and it's breaking CI on multiple repos. Hopefully they will sort it out so that when version 1.1.0 comes out, everything will be fixed again. Or lock version to 0.5.0.