Add possibility to configure annotations for statefulsets

[MatthiasWerning]

It would be very great to have something like a "commonAnnotations" value which content is applied to influxdb's statefulset. As there is currently no way to do that, we would have to clone and integrate the entire chart into our parent chart which is a mess just to change a few lines there.

For example the wave project for kubernetes requires a certain annotation on deployments / statefulsets in order to be active.