Revert "Temporarily switch system class submodule to github"

This reverts commit dde9e0b2a75d966dddb311fb51989313fdb5d1e3.

Change-Id: I09b2cea2edf955aa5e5144bceabf1dfd063068e6
diff --git a/.gitmodules b/.gitmodules
index 089f541..014c0c1 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "classes/system"]
 	path = classes/system
-	url = https://github.com/Mirantis/reclass-system-salt-model.git
+	url = https://gerrit.mcp.mirantis.net/salt-models/reclass-system