Override entrypoint

I tested the controller change by creating a release with the slugrunner image and a custom entrypoint of `["bash"]` and the process started ok, but the change to the cli does not work.

It looks like all the option parsing is broken as even `flynn run -r RELEASE COMMAND` doesn't work, it tries to execute with a `Cmd` of `["-r", "RELEASE", "COMMAND"]`.

I feel like I may be missing something, could someone test `flynn run -r RELEASE COMMAND`?