Support Downward API in Helm charts

[disrvptor]

• Added to envValuesFrom to helper
• Added to envValuesFrom to default values
• Updated readme files with the new environment block

Resolves #413

[philipp94831]

Suggested change

	{{ if .Values.envValueFrom }}
	{{- if .Values.envValueFrom }}

[philipp94831]

I think it would be better to have a map like approach here. So name is the key and valueFrom is the value. That way merging multiple value.yaml files is possible which is a pattern we often use

[philipp94831]

Suggested change

	{{ toYaml .Values.envValueFrom }}
	{{- range $key, $value := .Values.envValueFrom }}
	- name: {{ $key | quote }}
	valueFrom:
	{{ toYaml $value | indent 4 }}
	{{- end }}

should do the trick. values should look like this

envValueFrom: {}
#  NODE_NAME:
#    fieldRef:
#      fieldPath: spec.nodeName