Set cmd_env for trusty and xenial only (#573)

Only set the command environment for the supported releases which require it.

Co-authored-by: David Ames <[email protected]>

charmhelpers/fetch/ubuntu.py

@@ -650,14 +650,17 @@ def _add_apt_repository(spec):
     :param spec: the parameter to pass to add_apt_repository
     :type spec: str
     """
+    series = get_distrib_codename()
     if '{series}' in spec:
-        series = get_distrib_codename()
         spec = spec.replace('{series}', series)
     # software-properties package for bionic properly reacts to proxy settings
-    # passed as environment variables (See lp:1433761). This is not the case
-    # LTS and non-LTS releases below bionic.
-    _run_with_retries(['add-apt-repository', '--yes', spec],
-                      cmd_env=env_proxy_settings(['https', 'http']))
+    # set via apt.conf (see lp:1433761), however this is not the case for LTS
+    # and non-LTS releases before bionic.
+    if series in ('trusty', 'xenial'):
+        _run_with_retries(['add-apt-repository', '--yes', spec],
+                          cmd_env=env_proxy_settings(['https', 'http']))
+    else:
+        _run_with_retries(['add-apt-repository', '--yes', spec])
 
 
 def _add_cloud_pocket(pocket):