Sparkline chart tweaks

What does this MR do?

• Changes how GlSparklineCharts handles the variant prop (accepts a color name defined in scss_variables like gray900 vs. the actual value)
• Removes padding from the number that shows the latest y-value - spacing can be handled by the ui that integrates the chart
• Adds animation:true to series setting to prevent an issue with Safari that happens when the line is flat:

Kapture_2019-11-04_at_20.53.54