improve docker apm-server.yml management

[graphaelli]

#1543 introduced apm-server.docker.yml into the repo. We need to ensure that there is no drift between that and apm-server.yml, except where needed to support docker more naturally. Some options:

• generate apm-server.docker.yml during packaging
• add tests that only permit known difference between apm-server.yml and apm-server.docker.yml