title: Find route_table_id by subnet_id using boto3
body: <p>I'm trying to figure out how to find route_table_id based upon a subnet_id(in a vpc) using boto3. There is api in boto3(v1.3.1), ec2.RouteTableAssociation(id). With correct route_table_association_id, try to access the resource's attribute route_table_id, it gives exception as below:</p>
  
  <pre><code>In [19]: rta.route_table_id
  ---------------------------------------------------------------------------
  ResourceLoadException                     Traceback (most recent call last)
  &lt;ipython-input-19-a7f51e66ef03&gt; in &lt;module&gt;()
  ----&gt; 1 rta.route_table_id
  
  /usr/local/lib/python2.7/site-packages/boto3/resources/factory.pyc in property_loader(self)
      341                 else:
      342                     raise ResourceLoadException(
  --&gt; 343                         '{0} has no load method'.format(self.__class__.__name__))
      344
      345             return self.meta.data.get(name)
  
  ResourceLoadException: ec2.RouteTableAssociation has no load method
  </code></pre>
  
  <p>I'm looking for a clue why this failed.</p>
  
  <p>James</p>