title: CommandError: No model or app named in elastic search
body: <p>Here I am creating indices for a django model using django_elasticsearch_dsl but getting error while creating the one. My django app name where the code lies is example_app</p>
  
  <pre><code>django_elasticsearch_dsl = 0.5.1
  elasticsearch = 6.6.1
  </code></pre>
  
  <p>I have tried rebuilding indexes with some already existing indices that working prefectly fine and tried replacing the fields with the new model field and that also working fine. But when creating separate file throwing error.</p>
  
  <pre><code>from django_elasticsearch_dsl import DocType, Index, fields
  
  from example_app.models import RandomTable
  
  posts = Index('random_table')
  
  @posts.doc_type
  class ExampleDocument(DocType):
      class Meta:
          model = RandomTable
          fields = [
              'des',
              'quality',
              'con',
              'name',
              'prep',
              'sur_name',
              'issue_num'
            ]
  </code></pre>
  
  <p>when running the command python manage.py search_index --rebuild --models=example_app 
  Actual output is  CommandError: No model or app named example_app
  Expected output is creating index</p>