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/cli/rpc.py b/cli/rpc.py
index 4ac39a0..d97650b 100644
--- a/cli/rpc.py
+++ b/cli/rpc.py
@@ -25,12 +25,7 @@
             web_server = GLOBAL_CONFIG.get_config_value(
                     'SERVER', 'hostname', default=DEFAULT_SERVER)
 
-    # if the name doesn't start with http://,
-    # nonexistant hosts get an obscure error
-    if (not web_server.startswith('http://') and
-        not web_server.startswith('https://')):
-        web_server = 'http://' + web_server
-
+    web_server = rpc_client_lib.add_protocol(web_server)
     return web_server