Running `screenly edge-app run --generate-mock-data` should change `mock-data.yml` when the manifest file's schema changes even if it exists

[nicomiguelino]

• When screenly edge-app run --generate-mock-data runs, the CLI would complain that mock-data.yml already exists (if so).
• For instance, if I added, deleted, or modified (e.g., updating the default value of a setting) a setting in screenly.yml. Running screenly edge-app run --generate-mock-data should modify mock-data.yml.
• The --generate-mock-data option could be renamed to --sync-mock-data (or we could keep the former and introduce the latter as a drop-in replacement).