[pmm] add extraObjects

[TheRealNoob]

add extraObjects

extraObjects can be defined as strings or dictionaries. string format is useful if the field names are variable, for example...

extraObjects:
  - |
    apiVersion: v1
    kind: Secret
    type: Opaque
    data:
      {{- range $key, $value := .Values.someDict }}
        {{ $key }} : {{ b64enc $value }}
      {{- end }}

[it-percona-cla]

All committers have signed the CLA.

[skhtor]

Also eager for this to be merged. YAML merging is very useful and keeps our env-specific values files small.