Add support for iam update_user

This covers both the NewPath and NewUserName parameters for update_user, but
without regex validation for these values.

moto/iam/models.py

@@ -892,6 +892,18 @@ class IAMBackend(BaseBackend):
 
         return users
 
+    def update_user(self, user_name, new_path=None, new_user_name=None):
+        try:
+            user = self.users[user_name]
+        except KeyError:
+            raise IAMNotFoundException("User {0} not found".format(user_name))
+
+        if new_path:
+            user.path = new_path
+        if new_user_name:
+            user.name = new_user_name
+            self.users[new_user_name] = self.users.pop(user_name)
+
     def list_roles(self, path_prefix, marker, max_items):
         roles = None
         try:

moto/iam/responses.py

@@ -440,6 +440,18 @@ class IamResponse(BaseResponse):
         template = self.response_template(LIST_USERS_TEMPLATE)
         return template.render(action='List', users=users)
 
+    def update_user(self):
+        user_name = self._get_param('UserName')
+        new_path = self._get_param('NewPath')
+        new_user_name = self._get_param('NewUserName')
+        iam_backend.update_user(user_name, new_path, new_user_name)
+        if new_user_name:
+            user = iam_backend.get_user(new_user_name)
+        else:
+            user = iam_backend.get_user(user_name)
+        template = self.response_template(USER_TEMPLATE)
+        return template.render(action='Update', user=user)
+
     def create_login_profile(self):
         user_name = self._get_param('UserName')
         password = self._get_param('Password')