Use log scale for 5xx errors in "(cdn) HTTP Error Rate"

https://github.com/ripienaar/graphite-graph-dsl/wiki/Graphite-Render-API-Coverage
https://graphite.readthedocs.org/en/latest/render_api.html#logbase

Bug: 41754
Change-Id: I5d57accff0402daba0406ce0b21254cc0d644685
Also add 2 months and 1 year graphs in "(cdn) HTTP Error Rate"

For even better comparison. For instance, currently the graphs don't show
a single "normal" day.

Bug: 41754
Change-Id: I25793e531ccc6022ddcebb442f63a330e34d3cba