Mojo should include bundle.yaml in it's search path

Hi,

In a lot of places, we have the juju bundle named `bundle.yaml` rather than `bundle`. It would be nice if mojo could include that in it's search path per the error below:

| 2021-08-16 23:52:30 [ERROR] ("Config file 'bundle' not found.\nIs the MOJO_STAGE environment variable or the --stage option set?\nIs /home/stg-sentry/spec a valid spec?", ['bundle', 'staging/bundle', 'is/sentry/bundle', 'sentry/staging/bundle', 'is/sentry/staging/bundle'])
| Searched:
| bundle
| staging/bundle
| is/sentry/bundle
| sentry/staging/bundle
| is/sentry/staging/bundle

Thanks,

Haw