Allow cgi strategy for html and graphs

Activate cgi-generation of graphs (and html).

Instead of generating html and pngs all day and night long, it might be easier if that is just generated when someone is looking at it. (To solve the question: if a graph is generated but no one is there to look at it, is the graph really generated?)

So far I only used that together with apache.

@kampfschlaefer - This looks good. Can you please write a spec that proves it works as intended? I want to require this for all pull requests from now on.

Thanks!

Well, to prove that it works, one would have to start a machine in test-kitchen and run it for at least 10 minutes and then check that a) a http-request returns a valid page with images and b) /var/www/munin and/or /var/cache/munin/www stay empty as all html and png-graphs should be dynamic...

I think this can also be tested in Chefspec. When the graph strategy is specified as cgi then verify the config additions are written. If it's not, verify that they are not written.

I really want to get this in. Since you are busy I am going to write some tests for this.