Flaky test: temporarily disable checking some user_agent fields

[kaiyan-sheng]

Beats CI filebeat tests start failing caused by something changed with the user_agent plugin or regex, specifically user_agent.os.name, user_agent.name and user_agent.version field: https://travis-ci.org/elastic/beats/jobs/600418441#L2184

Removing some specific user_agent fields from checking instead of disabling the whole test_modules.py test.

Elasticsearch issue is created to track the real fix for the failing tests: elastic/elasticsearch#48318

[jsoriano]

An alternative to this: #14190, although files will need to be regenerated again if some changes are done to fix elastic/elasticsearch#48318

[kaiyan-sheng]

Actually I prefer Jaime's change #14190 in this case. So when the problem is fixed in ES, our CI will break again and we can regenerate the files to fix it right away.