elasticsearch: add known discrepancies to TestBeatsConfigSynced

[axw]

We have a test, TestBeatsConfigSynced, which ensures that the config for apm-server's elasticsearch client is a subset of libbeat's output.elasticsearch config.

Additions to libbeat config are not checked. Beats added Kerberos auth support, which we should mirror in our client (#3794). To make sure we keep on top of libbeat config changes, we should update the test to add a list of known fields in libbeat's config that we explicitly don't mirror.