Wait for app to start with torchx log (#763) (#763)

Summary:

Right now it silently times out after waiting for 10s and then (depending on the scheduler) fails because the job hasn't actually started; I'm trying to use `--log` with the `run` command and inevitably run into this.

I wasn't sure if we want to explicitly timeout, or what the best ergonomics would be -- I can increase the timeout to 10 minutes and blow up if it doesn't start in that time, or change it to a while loop (as done here) -- wanted to check :).

Differential Revision: D48840617