Add xpack.apm.enabled to whitelist

[sorenlouv]

APM in x-pack currently uses xpack.apm.ui.enabled but the plan is to move to xpack.apm.enabled for consistency with the other options. Since this is a breaking change we have to keep both until 7.0 before we can remove xpack.apm.ui.enabled.

cc @dadoonet

This is dependent on https://github.com/elastic/x-pack-kibana/pull/4489

[tylersmalley]

@sqren, outside of adding this to the docker whitelist xpack.apm.enabled should already work, right?

[sorenlouv]

@tylersmalley Ahh, I think you are right. Something just dawned on me - correct me if the following is wrong:

1. there is a global Kibana setting that interprets xpack.<plugin-name>.enabled (eg. the plugin doesn't have to actively add support for it)

2. The following is just a hack to hide APM from the menu - not actually disable APM https://github.com/elastic/x-pack-kibana/blob/2acc0ffc5c7733506c59f32b605726560c5b8a06/plugins/apm/index.js#L22-L28

So in short:

• I should remove xpack.apm.ui.enabled from this PR
• I should close this PR https://github.com/elastic/x-pack-kibana/pull/4489

[sorenlouv]

xpack.apm.ui.enabled was a hack we added for Cloud, so they can disable hide APM without triggering a restart of the server.

[tylersmalley]

@sqren, yup that's exactly correct. This PR LGTM and https://github.com/elastic/x-pack-kibana/pull/4489 can be closed since Kibana handles it internally.

[jarpy]

LGTM. Thanks for the help and persistence here.

[jarpy]

6.1 c918997
6.2 e63704c
6.x fc7d358