Simplify login form for oauth

(cherry picked from commit 89ba06d)
(cherry picked from commit 1dfc8f2)
(cherry picked from commit 7aefa26)
(cherry picked from commit e546132)

requirements.txt

@@ -27,6 +27,7 @@ flask-caching==1.4.0
 flask-compress==1.4.0
 flask-login==0.4.1        # via flask-appbuilder
 flask-migrate==2.1.1
+flask-oauthlib==0.9.5
 flask-openid==1.2.5       # via flask-appbuilder
 flask-sqlalchemy==2.3.2   # via flask-appbuilder, flask-migrate
 flask-wtf==0.14.2

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

@@ -0,0 +1,33 @@
+{#
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+#}
+{% 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 %}