Fix URLs to images to make them absolute

[nosnilmot]

This fixes display of images on "Graphs" page of local web UI, and
eliminates these errors from inets_error.log :

[25/Jan/2018:17:51:39 -0000] access to /es/images/graphes-cpu-mean_tn.png failed for 1.2.3.4, reason:
"mod_esi: Client not authorized to evaluate: images/graphes-cpu-mean_tn.png"

[highbass]

this fix needs to be merged... it resolved issues with graphs not being displayed for me.

@nniclausse could you take a look please.

[tisba]

I'm not sure how you are running into this issue. The tsung web UI does use graph_dy.thtml. I just gave this a try and I get the dygraph charts. Can you elaborate on how you've run into this issue?

Regarding your suggested changes: Making the image URLs absolute has the drawback, that you can no longer serve more then one report from a web server. If you have have multiple reports next to each other, then making the image paths absolute will have strange effects.

IMO for such reports it's a bad practise to assume that the report we are rending is the only webpage on this web server.

[nosnilmot]

this problem is now masked by PR #291 which enables use of --dygraph always - when --dygraph is not used, the image references were broken

[tisba]

Ah okay, I overlooked that. But I guess this issue can be closed then, right?