[dicuss] Support prepare and start dependencies

[v1v]

@dgieselaar mentioned that -start-opbeans-deps option is not implemented, as stated in the docs (see https://github.com/elastic/apm-integration-testing/blame/master/README.md#L100)

What he wanted is to start the apm-server with a postgres server.

So far the workaround is to start with the option --with-opbeans-python.

Let's discuss if this is something we can implement and use more generically for some other use cases. Or whether those dependencies are just dependencies and should not be exposed with any flag.