Use code from the distro package to improve linux distro detection. (#1629)

* Use code from the `distro` package to improve linux distro detection.

Various issues [1] [2] with the platform.linux_distribution function led
to the function being deprecated in python 3.4. The pipi package distro
[3] contains compatible improved code that better tracks current methods
for identifying linux distributions (see, for example, [4],[5]).

[1]: https://bugs.python.org/issue20454
[2]: http://bugs.python.org/issue1322
[3]: https://pypi.python.org/pypi/distro
[4]: https://refspecs.linuxbase.org/LSB_3.0.0/LSB-PDA/LSB-PDA/lsbrelease.html
[5]: https://www.freedesktop.org/software/systemd/man/os-release.html

* fix (potential) unicode problems

lib/spack/spack/operating_systems/linux_distro.py

@@ -1,5 +1,5 @@
 import re
-import platform as py_platform
+from external.distro import linux_distribution
 from spack.architecture import OperatingSystem
 
 
@@ -12,8 +12,9 @@ class LinuxDistro(OperatingSystem):
     """
 
     def __init__(self):
-        distname, version, _ = py_platform.linux_distribution(
+        distname, version, _ = linux_distribution(
             full_distribution_name=False)
+        distname, version = str(distname), str(version)
 
         # Grabs major version from tuple on redhat; on other platforms
         # grab the first legal identifier in the version field.  On