* Python3 Windows and Winlogbeat updates

This updates the Vagrantfile and addresses a few issues. I installed Python3 on Windows
via chocolotey and the python MSI and neither provided a `python3` command so I adjusted
the build tooling to use `python` on Windows.

Another issue I hit was that under Vagrant with the shared filesystem, `pip` would not
install into the virtual environment. It only will install when moving the PYTHON_ENV
off of the shared filesystem. So when provisining the Vagrant VM it will now put
PYTHON_ENV in C:\beats-python-env. Remember this if you need need to do a completely
clean build on Windows+Vagrant.

There were some encoding issues I found while reading output files containing unicode.
So I modified the libbeat's beat.py to specify `utf_8` encoding when opening output files.
I was seeing errors like:

```
======================================================================
ERROR: eventlogging - UTF-16 characters
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Gopath\src\github.com\elastic\beats\winlogbeat\tests\system\test_eventlogging.py", line 187, in test_utf16_characters
    evts = self.read_events(config={
  File "C:\Gopath\src\github.com\elastic\beats\winlogbeat\tests\system\winlogbeat.py", line 110, in read_events
    return self.read_output()
  File "C:\Gopath\src\github.com\elastic\beats\winlogbeat\tests\system\../../../libbeat/tests/system\beat\beat.py", line 260, in read_output
    for line in f:
  File "C:\Python38\lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x8d in position 277: character maps to <undefined>
```

I bumped the Pillow version in the pip requirements because the newer version provides a
precompiled wheel for Python 3 on Windows. This avoids having to compiling it. I bumped
the Docker versions while debugging and just left them at the latest version since nothing broke.

I tested Winlogbeat and Metricbeat's `mage pythonUnitTest` on Windows 2019 and all tests passed.