Default DataGateway is set to localhost, causing file uploads to fail

[SamuAlfageme]

In the context of sciencemesh/sciencemesh#105 and while testing the wopiserver deployment together with a ruuning Reva and its simplified config from #826 (standalone.toml), we hit an error when trying to upload a file to Reva from the wopiserver integration tests:

2020-07-01T15:23:53 wopiserver.test[35] ERROR    
  msg="Exception when uploading file to Reva" 
  reason="HTTPConnectionPool(host='localhost', port=19001): Max retries exceeded with url: /data (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fd1b0677e80>: Failed to establish a new connection: [Errno 111] Connection refused'))"

By looking at the revad logs:

3:23PM DBG internal/grpc/interceptors/auth/auth.go:85 > skipping auth method=/cs3.gateway.v1beta1.GatewayAPI/InitiateFileUpload pid=1 pkg=rgrpc traceid=98f4c5d19ca44fc77a9292f213a1f3a3
3:23PM INF internal/grpc/interceptors/log/log.go:69 > unary code=OK end="01/Jul/2020:15:23:53 +0000" from=tcp://127.0.0.1:34036 pid=1 pkg=rgrpc start="01/Jul/2020:15:23:53 +0000" time_ns=102801 traceid=98f4c5d19ca44fc77a9292f213a1f3a3 uri=/cs3.storage.registry.v1beta1.RegistryAPI/GetStorageProvider user-agent=grpc-go/1.26.0
3:23PM DBG pkg/storage/utils/localfs/upload.go:124 > localfs: NewUpload info={"ID":"","IsFinal":false,"IsPartial":false,"MetaData":{"dir":"/","filename":"testxattr.txt"},"Offset":0,"PartialUploads":null,"Size":0,"SizeIsDeferred":false,"Storage":null} pid=1 pkg=rgrpc traceid=98f4c5d19ca44fc77a9292f213a1f3a3
3:23PM DBG pkg/storage/utils/localfs/upload.go:140 > localfs: resolved filename info={"ID":"","IsFinal":false,"IsPartial":false,"MetaData":{"dir":"/","filename":"testxattr.txt"},"Offset":0,"PartialUploads":null,"Size":0,"SizeIsDeferred":false,"Storage":null} pid=1 pkg=rgrpc traceid=98f4c5d19ca44fc77a9292f213a1f3a3
3:23PM INF internal/grpc/services/storageprovider/storageprovider.go:302 > file upload data-server=http://localhost:19001/data/654f6dc4-375c-4e94-ae95-6cb5e6bdd9c6 fn=/testxattr.txt pid=1 pkg=rgrpc traceid=98f4c5d19ca44fc77a9292f213a1f3a3 xs="map[md5:100 unset:1000]"
3:23PM INF internal/grpc/interceptors/log/log.go:69 > unary code=OK end="01/Jul/2020:15:23:53 +0000" from=tcp://127.0.0.1:34038 pid=1 pkg=rgrpc start="01/Jul/2020:15:23:53 +0000" time_ns=1554022 traceid=98f4c5d19ca44fc77a9292f213a1f3a3 uri=/cs3.storage.provider.v1beta1.ProviderAPI/InitiateFileUpload user-agent=grpc-go/1.26.0
3:23PM INF internal/grpc/interceptors/log/log.go:69 > unary code=OK end="01/Jul/2020:15:23:53 +0000" from=tcp://10.100.69.158:33074 pid=1 pkg=rgrpc start="01/Jul/2020:15:23:53 +0000" time_ns=3607498 traceid=98f4c5d19ca44fc77a9292f213a1f3a3 uri=/cs3.gateway.v1beta1.GatewayAPI/InitiateFileUpload user-agent="grpc-python/1.30.0 grpc-c/10.0.0 (manylinux; chttp2)"

... we realized the Datagateway is using localhost as fallback when no url is provided in

reva/pkg/sharedconf/sharedconf.go

Lines 44 to 47 in e8f1815

	// this is the default address we use for starting HTTP services
	if sharedConf.DataGateway == "" {
	sharedConf.DataGateway = "http://localhost:19001/data"
	}

For some better and more flexible defaults, we've discussed the option to include a 2nd fallback: the hostname, which e.g. in kubernetes will resolve based on https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/#pod-s-hostname-and-subdomain-fields - it won't be as "correct" as the (kubernetes) service name, but will point at the right instance. The preference could be:

1. Preferred the provided value from the config file, if any.
2. Revad's hostname, if set.
3. Fallback to localhost.