Run fluentd as user by default

[pepov]

Is your feature request related to a problem? Please describe.
Fluentd currently runs with root which is unreasonable from a security perspective.

Describe the solution you'd like
Fluentd should run as the fluentd user which is already configured in the image.

Describe alternatives you've considered
Make this a documented option, but leave it as is.

Additional context
We need to consider if this might break existing deployments, but hopefully it shouldn't.