Fix platform detection in case of FreeBSD in compat module.

[dwich]

Problem: For platform detection for Python >= 3.6 the distro package is used. For lower version of Python the built-in package platform is used.

distro.linux_distribution() is used to detect Linux
distro.version() is used to detecting FreeBSD

Method distro.version() should not be used because distro can be either distro itself or import platform as distro, therefore:

platform.version() and distro.version() return different values.
platform.version() returns 'FreeBSD 11.2-RELEASE-p9.....'
distro.version() returns '11.2'

Solution: Do not detect platform and version (in case of FreeBSD) using distro package, which is useful for linux but it's not necessary for FreeBSD.

[codecov]

Codecov Report

Merging #756 into development will decrease coverage by <.01%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           development     #756      +/-   ##
===============================================
- Coverage        97.76%   97.76%   -0.01%     
===============================================
  Files               62       62              
  Lines             4528     4527       -1     
  Branches           232      232              
===============================================
- Hits              4427     4426       -1     
  Misses              86       86              
  Partials            15       15

Impacted Files	Coverage Δ
...-engine/google_compute_engine/tests/compat_test.py	84.09% <100%> (-0.7%)	⬇️
...gle-compute-engine/google_compute_engine/compat.py	76.71% <100%> (+0.32%)	⬆️

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[hopkiw]

Confirmed that there's no variance distro vs platform on supported Linux distributions and that we never expect/utilize version info for FreeBSD. Logic looks fine for current applications.

[illfelder]

Thanks for the contribution! This looks good to me.