Use slug in GitHub dex config

[mogottsch]

This PR updates the default GitHub connector config to include teamNameField: slug.
See the dex docs for details.

As the only team we are currently using is giantswarm-admins, whose name and slug are equal, this won't have any effect.

Towards giantswarm/roadmap#1205

Note

I've also changed the yaml marshalling. The config is converted to JSON first, so that the json annotations on the config struct are actually used and not just a lower-case representation of the key's name.

before:

- type: github
  id: giantswarm-github
  name: Github for Giant Swarm
  config:
    org: ""
    orgs:
      - name: giantswarm
        teams:
          - giantswarm-admins
    hostname: ""
    rootca: ""
    teamnamefield: ""

after:

- type: github
  id: giantswarm-github
  name: Github for Giant Swarm
  config:
    hostName: ""
    org: ""
    orgs:
      - name: giantswarm
        teams:
          - giantswarm-admins
    rootCA: ""
    teamNameField: slug

[vvondruska]

Looks good to me.
Personally I am not aware of any other way to marshal YAML using JSON tags.