Cherry-pick #16235 to 7.x: Freeze virtualenv version until issue with CI is resolved

[michalpristas]

Cherry-pick of PR #16235 to 7.x branch. Original message:

CI when fetching new version of virtualenv (20.0.0) is failing on

Generated fields.yml for filebeat to /Users/travis/gopath/src/github.com/elastic/beats/filebeat/build/fields/fields.all.yml
Traceback (most recent call last):
  File "/Users/travis/gopath/src/github.com/elastic/beats/build/ve/darwin/bin/pip", line 6, in <module>
    from pip._internal.cli.main import main
ImportError: No module named main
Error: running "/Users/travis/gopath/src/github.com/elastic/beats/build/ve/darwin/bin/pip install --quiet -Ur /Users/travis/gopath/src/github.com/elastic/beats/libbeat/tests/system/requirements.txt" failed with exit code 1
make: *** [update] Error 1

This PR just freezes version so we can get a green run, issue with new virtualenv still needs to be solved

[ycombinator]

Travis CI is green and Jenkins CI failures are unrelated. Merging.