Presents color contrast in resource usage charts

This makes use of d3.scale.category10() to allocate colors to per tenant
trend lines as opposed to d3.scale.category20().
This makes it easy to distinguish the color rendered for each new tenant as
d3.scale.category10() is a more contrasty.

Closes-Bug: #1243747

Change-Id: I6ac2bcd4fa831944dfb10944eb620d333e1f3644
Merge "Presents color contrast in resource usage charts"