Allow detection of Clear Linux OS in python3

In python3 the platform.linux_distribution() function does not display
content for Clear Linux. In addition the linux_distribution function
is due to be deprecated in python 3.7 (the next minor release, see
https://bugs.python.org/issue1322). In anticipation of this, switch to
sourcing the os-release file in the waagent.service and reading the
distribution information from the environment instead.

azurelinuxagent/common/version.py

@@ -86,9 +86,23 @@ def get_distro():
                                                   supported_dists=supported))
         full_name = platform.linux_distribution()[0].strip()
         osinfo.append(full_name)
-    else:
+    elif 'dist' in dir(platform):
         osinfo = platform.dist()
 
+    # In the case that platform.dist doesn't provide this information
+    # (see https://bugs.python.org/issue1322)
+    # take the details from the environment for Clear Linux (as the
+    # waagent.service will add them).
+    #
+    # Eventually consider using a library to handle other distros like:
+    # https://pypi.python.org/pypi/distro which is built to do this as
+    # the stdlib folks prefer this functionality live in pypi.
+    if not osinfo or osinfo[0] == '' and os.environ.get('NAME'):
+        osinfo = [os.environ['NAME'],
+                  os.environ['VERSION_ID'],
+                  os.environ['ID'],
+                  os.environ['PRETTY_NAME']]
+
     # The platform.py lib has issue with detecting oracle linux distribution.
     # Merge the following patch provided by oracle as a temporary fix.
     if os.path.exists("/etc/oracle-release"):

init/clearlinux/waagent.service

@@ -8,6 +8,7 @@ ConditionPathExists=/usr/share/defaults/waagent/waagent.conf
 
 [Service]
 Type=simple
+EnvironmentFile=/usr/lib/os-release
 ExecStart=/usr/bin/python -u /usr/bin/waagent -daemon
 Restart=always
 RestartSec=5