Introduce a config file to cellxgene (#1264)

* Introduce a config file to cellxgene

The config file format is in yaml.  The default config is located
in server/common/default_config.py.  A user may create a yaml file
that contains a subset of these fields.  It can be used during cellxgene
launch, or for hosted cellxgene.

The code has also been refactored.  Much of the logic to check arguments
has moved from launch to app config.

It is now possible to set the tiledb context parameters using the config
file.  Other feature will soon be handled in a similar way.

server/eb/README.md

@@ -40,6 +40,10 @@ There are many more options to these commands that may be important or necessary
 
 3. Create the artifact.zip file for the application
 
+    If you have additional configuration for the application,
+    place that application in server/eb/config.yaml.  This will
+    be included in the eb deployment.
+    
    ```
    make build
    ```