Option to load Envoy config from external file

[kondratyevd]

This PR addresses #80

New parameters allow to load Envoy config from an arbitrary ConfigMap in the installation namespace. ConfigMap name and key of the file stored in it must be specified.

Example:

envoy:
  external_config:
    load_from_configmap: true
    configmap_name: external-envoy-config
    configmap_key: envoy.yaml

[kondratyevd]

@briedel what do you think?

[briedel]

Looks good to me.

[kondratyevd]

Okay I'll add some documentation and then merge