Add Google SSO

This pull request adds Google SSO support. There are two main changes:

1. Add the Google auth view resource
2. Make passwords optional when creating a new user. This allows an admin
to create a user without a password so that they can only login via Google.

lemur/users/models.py

@@ -52,7 +52,10 @@ class User(db.Model):
         :param password:
         :return:
         """
-        return bcrypt.check_password_hash(self.password, password)
+        if self.password:
+            return bcrypt.check_password_hash(self.password, password)
+        else:
+            return False
 
     def hash_password(self):
         """
@@ -60,8 +63,11 @@ class User(db.Model):
 
         :return:
         """
-        self.password = bcrypt.generate_password_hash(self.password)
-        return self.password
+        if self.password:
+            self.password = bcrypt.generate_password_hash(self.password)
+            return self.password
+        else:
+            return None
 
     @property
     def is_admin(self):

lemur/users/views.py

@@ -157,7 +157,7 @@ class UsersList(AuthenticatedResource):
         """
         self.reqparse.add_argument('username', type=str, location='json', required=True)
         self.reqparse.add_argument('email', type=str, location='json', required=True)
-        self.reqparse.add_argument('password', type=str, location='json', required=True)
+        self.reqparse.add_argument('password', type=str, location='json', default=None)
         self.reqparse.add_argument('active', type=bool, default=True, location='json')
         self.reqparse.add_argument('roles', type=roles, default=[], location='json')