fix(dashboards): use new grafonnet for apiserver dashboard

[Duologic]

ref: https://github.com/kubernetes-monitoring/kubernetes-mixin/issues/888

This updates the apiserver dashboard to use the new Grafonnet.

This will cause some changes but should result in the same dashboard.

A few noteable changes:

• Less 'defaults' are set, grafonnet-lib was quite verbose, the new grafonnet only adds the necessary.
• deprecated graph panel is replaced by timeSeries panel
• deprecated singlestat panel is replaced by the stat panel
• the 'row' structure is now calculated on the grid based on the width of the panels, it should match the original layout

The code abstracts a few of these panels into separate functions to avoid duplication a bit.

Also see inline comments about a few changes outside the dashboard.

[povilasv]

Thanks for this, rebase needed :)

[Duologic]

Rebased, dropped a few commits.