[nginx] add support for Services with ExternalNames

[aledbf]

(services with no endpoints)

type: ExternalName
externalName: api.calculator.com

from https://medium.com/@jeffzzq/modeling-environments-with-linkerd-ingress-part-3-3-3b8beda4a4bd

[gianrubio]

@aledbf could we use this for dns name and ip address ? Few days ago someone asked on slack about this, this can be useful for Chinese proxying.

[aledbf]

@gianrubio please check #621

[gianrubio]

This keep working only for dns name, looking in the k8s types it's not possible to do with an external ip address.

Do you thinks this service make sense?

kind: Service
apiVersion: v1
metadata:
  name: proxy-google-cl
spec:
  ports:
  - protocol: TCP
    port: 80
    targetPort: 80
  type: ExternalIp
  externalIp: 8.8.8.8 

[aledbf]

@gianrubio makes sense but the externalIP will work only if the server has no "virtual" servers/host (that is the reason why I added the snippet annotation to be able to reach google.com)