Temporarily switch system class submodule to github

While we're awating for renewal of certificate for Gerrit, let's
switch system submodule to Github, to unblock deployment jobs.

Change-Id: Ia325308242933162aaab798e7b2229cd502494a1
diff --git a/.gitmodules b/.gitmodules
index 014c0c1..089f541 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "classes/system"]
 	path = classes/system
-	url = https://gerrit.mcp.mirantis.net/salt-models/reclass-system
+	url = https://github.com/Mirantis/reclass-system-salt-model.git