title: change model admin form default value
body: <p>I have typical model inheritance in my project:</p>
  
  <pre><code>class A(models.Model):
      boolean_field = models.BooleanField(default=True)
  
  class B(A):
      some_other_field = models.CharField()
  </code></pre>
  
  <p>I want to override default value of <code>boolean_field</code> in class <code>B</code>, how can I do it?</p>
  
  <p>I think that could be tricky thing to do on a database layer, so maybe at least I can simply override that default value in Django admin (I mean in <code>ModelAdmin</code> form for class <code>B</code>).</p>