[mattermost-enterprise-edition] Allow existing config.json

[Moep90]

Allow to use an existingConfigJson instead of the predefined tmpl or alternativly allow to configure every section with defaults.
like:

global:
  TeamSettings:
    enableTeamCreation: true
    enableUserCreation: true
    enableOpenServer: true
.......
  ServiceSettings:

    # already existing
    enableLinkPreviews: true
    enableCustomEmoji: true
    
  # example additional values
    GoogleDeveloperKey: ""
    EnableOAuthServiceProvider: false
    EnableIncomingWebhooks: true
    EnableOutgoingWebhooks: true
    EnableCommands: true
    EnableOnlyAdminIntegrations: false
    EnablePostUsernameOverride: false
    EnablePostIconOverride: false
.....

....and make use of helm flow control
Docs
StackOverflow

For example:

  {{- range $v := .Values.global.TeamSettings }}
....
  {{- end }}

or

global:
  existingConfigmapJsonName: myconfigmapjson
# or
global:
  existingConfigmapJson:
    name: myconfigmapjson