[AIRFLOW-XXX] Better instructions for airflow flower (#4214)

* Better instructions for airflow flower

It is not clear in the documentation that you need to have flower installed to successful run airflow flower. If you don't have flower installed, running airflow flower will show the following error which is not of much help:

airflow flower                                                                                       
[2018-11-20 17:01:14,836] {__init__.py:51} INFO - Using executor SequentialExecutor                                                      
Traceback (most recent call last):                                                                                                       
  File "/mnt/secondary/workspace/f4/typo-backend/pipelines/model-pipeline/airflow/bin/airflow", line 32, in <module>                     
    args.func(args)                                                                                                                      
  File "/mnt/secondary/workspace/f4/typo-backend/pipelines/model-pipeline/airflow/lib/python3.6/site-packages/airflow/utils/cli.py", line
 74, in wrapper                                                                                                                          
    return f(*args, **kwargs)                                                                                                            
  File "/mnt/secondary/workspace/f4/typo-backend/pipelines/model-pipeline/airflow/lib/python3.6/site-packages/airflow/bin/cli.py", line 1
221, in flower                                                                                                                           
    broka, address, port, api, flower_conf, url_prefix])                                                                                 
  File "/mnt/secondary/workspace/f4/typo-backend/pipelines/model-pipeline/airflow/lib/python3.6/os.py", line 559, in execvp              
    _execvpe(file, args)                                                                                                                 
  File "/mnt/secondary/workspace/f4/typo-backend/pipelines/model-pipeline/airflow/lib/python3.6/os.py", line 604, in _execvpe            
    raise last_exc.with_traceback(tb)                                                                                                    
  File "/mnt/secondary/workspace/f4/typo-backend/pipelines/model-pipeline/airflow/lib/python3.6/os.py", line 594, in _execvpe            
    exec_func(fullname, *argrest)                                                                                                        FileNotFoundError: [Errno 2] No such file or directory

* Update use-celery.rst

docs/howto/executor/use-celery.rst

@@ -57,6 +57,13 @@ Note that you can also run "Celery Flower", a web UI built on top of Celery,
 to monitor your workers. You can use the shortcut command ``airflow flower``
 to start a Flower web server.
 
+Please note that you must have the ``flower`` python library already installed on your system. The recommend way is to install the airflow celery bundle.
+
+.. code-block:: bash
+
+    pip install 'apache-airflow[celery]'
+
+
 Some caveats:
 
 - Make sure to use a database backed result backend