helm chart: expose devRoot

[jgehrcke]

devRoot defaults to ${nvidiaDriverRoot}/dev.

In certain environments we have to be able to set devRoot independent of nvidiaDriverRoot. Currently that is seemingly not exposed.

Will need to investigate further.

[jgehrcke]

We picked a different solution for the environment that motivated this idea. Also see #307.

Let's close this for now.

(keeping the set of exposed parameters at the absolute required minimum is an important goal to keep in mind)