aci: accessing private docker registry on non-standard port with ACI

Ported from [user feedback]( https://social.msdn.microsoft.com/Forums/en-US/8eda3544-53ed-43fa-ac53-87e721fa09c3/accessing-private-docker-registry-on-nonstandard-port-with-aci?forum=AzureContainerServices).

```
az container create\
--name testcontainer\
--resource-group test-rg
--image private.example.com:5000/dockerimage:build94\
--ports 80\
--ip-address Private\
--registry-login-server private.example.com:5000\
--registry-username reguser\
--registry-password ***
```
fails with an error of
`The server 'private.example.com:5000' in the 'imageRegistryCredentials' of container group 'testcontainer' is invalid. It should be a valid host name without protocol`

[Linked issue](https://github.com/Azure/acr/issues/287) at ACR was closed as we determined the issue is no longer ACR related.

//cc: @kawaja
//cc: @mmacy, @neilpeterson, @seanmck @dlepow