title: Override S3 endpoint using Boto3 configuration file
body: <h3>OVERVIEW:</h3>
  <p>I'm trying to override certain variables in <code>boto3</code> using the configuration file (<code>~/aws/confg</code>).
  In my use case I want to use <code>fakes3</code> service and send S3 requests to the localhost.</p>
  <h3>EXAMPLE:</h3>
  <p>In <code>boto</code> (not <code>boto3</code>), I can create a config in <code>~/.boto</code> similar to this one:</p>
  <pre><code>[s3]
  host = localhost
  calling_format = boto.s3.connection.OrdinaryCallingFormat
  
  [Boto]
  is_secure = False
  </code></pre>
  <p>And the client can successfully pick up desired changes and instead of sending traffic to the real S3 service, it will send it to the localhost.</p>
  <pre><code>&gt;&gt;&gt; import boto
  &gt;&gt;&gt; boto.connect_s3()
  S3Connection:localhost
  &gt;&gt;&gt; 
  </code></pre>
  <h3>WHAT I TRIED:</h3>
  <p>I'm trying to achieve a similar result using <code>boto3</code> library. By looking at the source code I found that I can use <code>~/aws/config</code> location. I've also found an example config in <code>unittests</code> folder of <code>botocore</code>.</p>
  <p>I tried to modify the config to achieve the desired behaviour.  But unfortunately, it doesn't work.</p>
  <p>Here is the config:</p>
  <pre><code>[default]
  aws_access_key_id = XXXXXXXXX
  aws_secret_access_key = YYYYYYYYYYYYYY
  region = us-east-1
  is_secure = False
  s3 =
      host = localhost
  </code></pre>
  <h3>QUESTION:</h3>
  <ol>
  <li>How to overwrite <code>clients</code> variables using config file?</li>
  <li>Where can I find a complete list of allowed variables for the configuration?</li>
  </ol>