Fluentd: Allow users to override pod entrypoint

[Boojapho]

In the fluentd chart, there is no way to override the default entrypoint of the image. It either uses the default or is hard coded to /fluentd/entrypoint.sh (if using plugins). Provide a value to override the entrypoint, with a default of /fluentd/entrypoint.sh.

My suggestion is to always use the pattern:

command:
- "/bin/sh"
- "-c"
- |
  {{- range $plugin := .Values.plugins }}
    {{- print "fluent-gem install " $plugin | nindent 6 }}
  {{- end }}
  exec {{ .Values.podEntrypoint }}

Without plugins, the entrypoint will replace the shell. With plugins, the plugins will install, then the entrypoint will replace the shell. This also allows us to pass additional arguments to the entrypoint without making it into an array.