Simplify login form for oauth

(cherry picked from commit 89ba06d9a6)

superset/templates/appbuilder/general/security/login_oauth.html

@@ -0,0 +1,15 @@
+{% extends "appbuilder/base.html" %}
+
+{% block content %}
+
+<div class="container">
+  <div id="loginbox" style="margin-top:50px;" class="mainbox col-md-6 col-md-offset-3 col-sm-8 col-sm-offset-2">
+    <center>
+      <a href="/login/google">
+        <img width="300" src="https://developers.google.com/accounts/images/sign-in-with-google.png">
+      </a>
+    </center>
+  </div>
+</div>
+
+{% endblock %}