Salt minion allow to specify HTTP backend

This is useful when using proxies. The default Tornado backend does not
utilize proxy environment variables and isn't able to set no_proxy variable.

Change-Id: I4a51c6fc9abe65d46ed4f3adeb30f7a25337857e

.kitchen.yml

         minion_handler.sls: tests/pillar/minion_custom_handler.sls
         minion_local_pillar.sls: tests/pillar/minion_local_pillar.sls
         minion_local_reclass.sls: tests/pillar/minion_local_reclass.sls
+        minion_backend_urllib.sls: tests/pillar/minion_backend_urllib.sls
 
   - name: master-default
     provisioner:

README.rst

           host: 127.0.0.1
           port: 3128
 
+Salt minion to specify non-default HTTP backend. The default tornado backend
+does not respect HTTP proxy settings set as environment variables. This is
+useful for cases where you need to set no_proxy lists.
+
+.. code-block:: yaml
+
+    salt:
+      minion:
+        backend: urllib2
+
+
 Salt minion with PKI certificate authority (CA)
 
 .. literalinclude:: tests/pillar/minion_pki_ca.sls

salt/files/minion.conf

 proxy_port: {{ minion.proxy.port }}
 {%- endif %}
 
+{%- if minion.backend is defined %}
+backend: {{ minion.backend }}
+{%- endif %}
+
 {%- if minion.sentry is defined %}
 sentry_handler:
 {% for server in minion.sentry.servers %}

tests/pillar/minion_backend_urllib.sls

+salt:
+  minion:
+    enabled: true
+    backend: urllib2