Add flag to disable checking if configured backends are up

[williamoverton]

When rapidly developing (especially on questionable wifi), waiting for the CLI to verify the backend every time is frustrating. Could we add a flag such as "--skip-backend-checking" to skip this behaviour? (or a shorter flag as that is a little wordy)

✓ Running local server...

Oct 17 11:36:58.724  INFO checking if backend 'aws_origin' is up
Oct 17 11:36:58.871  INFO backend 'aws_origin' is up
Oct 17 11:36:58.872  INFO checking if backend 'product_origin' is up
Oct 17 11:36:58.899  INFO backend 'product_origin' is up
Oct 17 11:36:58.899  INFO Listening on http://127.0.0.1:7676

[GeeWee]

This would also be helpful for some of our CI / testing setup.