Graph the max value rather than avg if rrdsingle disabled

Graph the max value rather than avg if rrdsingle disabled

With rrdsingle the average values are used for graphing. As the
timescale increases (say over a day or month) the average values
progressively reduce.  Using the maximum value over the longer periods
is arguably no less deceptive but it does produce more informative
graphs.

V2 - Invert the default path logic related to rra.single

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>
(cherry picked from commit f960a5286896aacbd95a7b95aa367d1584106b2b)