Fix pip install error

If we set mutiple proxy (ip or url), pip install will
treat the second proxy as a command.
Add quotation marks around proxy.
expecially for no_proxy

Change-Id: I38ad3f083ba5155cda0e5e2b8f5df64492b7fecd
diff --git a/inc/python b/inc/python
index d72c3c9..229c540 100644
--- a/inc/python
+++ b/inc/python
@@ -94,9 +94,9 @@
 
     $xtrace
     $sudo_pip \
-        http_proxy=${http_proxy:-} \
-        https_proxy=${https_proxy:-} \
-        no_proxy=${no_proxy:-} \
+        http_proxy="${http_proxy:-}" \
+        https_proxy="${https_proxy:-}" \
+        no_proxy="${no_proxy:-}" \
         PIP_FIND_LINKS=$PIP_FIND_LINKS \
         $cmd_pip install \
         $@