autotest: Respect url protocol in RPC client library.

BUG=chromium:795874
TEST=Manually run atest and dut-status commands against http and https
     servers.

Change-Id: I4b609101f3d48cd46c766c04d72ac9a6a3299f8b
diff --git a/server/frontend.py b/server/frontend.py
index bfcba8f..49d1b04 100644
--- a/server/frontend.py
+++ b/server/frontend.py
@@ -90,7 +90,7 @@
         self.debug = debug
         self.reply_debug = reply_debug
         headers = {'AUTHORIZATION': self.user}
-        rpc_server = 'http://' + server + path
+        rpc_server = rpc_client_lib.add_protocol(server) + path
         if debug:
             print 'SERVER: %s' % rpc_server
             print 'HEADERS: %s' % headers