Specify a file name in the configuration for GCS Write

[kolban-google]

Currently, when we use the GCS Write node, we expect to find a file name in the msg.filename property at runtime. While this is great, we can do more. This issue suggests a new feature that provides an optional file name in the configuration properties for the node. The suggested semantics are:

• If msg.filename exists ... use it
• if msg.filename does NOT exist then use the configured filename supplied by the flow designer
• If neither msg.filename nor a configured filename exist, then that would be an error.

[kolban-google]

The code changes have been made and pushed to the dev branch. This issue will be closed once published for download.

[DavidGasca]

Moved to its own issue ... #5