globus flows templatize flow definitions

## Customer Story
As a flow author, I'd like to be able to submit a templatized version of a flow definition which has values injected at runtime. Those values should be configurable via environment variable on submission (both via create_flow and update_flow).

This would enable me to persist my flow definitions outside of globus (ie. in a github repo) and inject private parameters like an email server secret as deployment time.