Update for app config (#1369)

fixes an issue with "cellxgene launch" which had a bad interaction between
command line parameters and config file parameters.
Now, the config files are applied first, followed by the parameters that
were provided in the command line.

There is also now a check that each of the config attributes is type checked.

server/app/app.py

@@ -124,7 +124,7 @@ def dataroot_test_index():
     data += "<body><H1>Welcome to cellxgene</H1>"
 
     config = current_app.app_config
-    locator = DataLocator(config.multi_dataset__dataroot, config=config)
+    locator = DataLocator(config.multi_dataset__dataroot, app_config=config)
     datasets = []
     for fname in locator.ls():
         location = path_join(config.multi_dataset__dataroot, fname)