make install-mongodb doesn't complete on macos

[tobybellwood]

Owing to the image used in the chart bitnami/mongodb:12.1.31 not supporting arm64 (none of the https://hub.docker.com/r/bitnami/mongodb ones do) this chart never installs - and the only way to progress with the install on MacOS is to scale the deployment to 0 during install.

We need to either:

• find a replacement drop-in image that works in macos
• replace the chart
• make the chart skip on macos

[shreddedbacon]

We should do this for mariadb, postgres, and mongodb as being optional in a local-stack but required for tests?

• make the chart skip on macos