Adding environment variables for salt minion

salt-minion started via upstart is not using /etc/environment to
source variables therefore is not using system wide http_proxy parameters if
they are set there. With this patch if salt.minion.env_vars is set to engine:
file salt will set envvars in /etc/default/salt-minion and will use them for
all commands executed remotely via salt. Since /etc/default is not used on
systems with systemd override for salt-minion to use /etc/environmnet will be
set.

Change-Id: Icfe122de2486a83820a0bd2e2aeea4be3796c6aa
Closes-bug: PROD-16676
diff --git a/README.rst b/README.rst
index c4a41f8..d8d8674 100644
--- a/README.rst
+++ b/README.rst
@@ -521,6 +521,15 @@
       minion:
         backend: urllib2
 
+Salt minion using environment variables defined in /etc/default/salt-minion for
+upstart based Ubuntu and in /etc/environment for Ubuntu with systemd.
+
+.. code-block:: yaml
+
+    salt:
+      minion:
+        env_vars:
+          engine: file
 
 Salt minion with PKI certificate authority (CA)