Simplify how we specify the list of available executors to execute_run_host_mode

Summary: In hindsight I'm not sure why we went with the original setup where we roll our own crappy config schema. Use more of the existing machinery for turning a list of executor defs into an Executor.

Test Plan: BK

Reviewers: alangenfeld, johann

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D8284