celery: introduce make_app instead of creating app at import time

It is dirty to instantiate things at import time (unless it really is basic
singletons).

In 0.5.1 (and earlier), such dirtyness made partial test execution fail when
other things had global side effects and things didn't use the usual import
order:

    $ py.test kallithea/lib/
    collecting ...
    ――― kallithea/lib/celerypylons/__init__.py ―――
    kallithea/lib/celerypylons/__init__.py:58: in <module>
        app.config_from_object(celery_config(tg.config))
    kallithea/lib/celerypylons/__init__.py:28: in celery_config
        assert config['celery.imports'] == 'kallithea.lib.celerylib.tasks', 'Kallithea Celery configuration has not been loaded'
    data/env/lib/python2.7/site-packages/tg/configuration/tgconfig.py:31: in __getitem__
        return self.config_proxy.current_conf()[key]
    E   KeyError: 'celery.imports'

Avoid that by running a "factory" function when the celery app actually is
needed.

kallithea/bin/kallithea_cli_celery.py

@@ -16,6 +16,7 b' import click'
 
 import kallithea
 import kallithea.bin.kallithea_cli_base as cli_base
+from kallithea.lib import celerypylons
 
 
 @cli_base.register_command(config_file_initialize_app=True)
@@ -35,6 +36,6 b' def celery_run(celery_args):'
         raise Exception('Please set use_celery = true in .ini config '
                         'file before running this command')
 
-    from kallithea.lib import celerypylons
-    cmd = celerypylons.worker.worker(celerypylons.app)
+    app = celerypylons.make_app()
+    cmd = celerypylons.worker.worker(app)
     return cmd.run_from_argv(None, command='celery-run -c CONFIG_FILE --', argv=list(celery_args))

kallithea/lib/celerypylons/__init__.py

@@ -53,6 +53,8 b' def celery_config(config):'
     return celery_config
 
 
-# Create celery app from the TurboGears configuration file
-app = celery.Celery()
-app.config_from_object(celery_config(tg.config))
+def make_app():
+    """Create celery app from the TurboGears configuration file"""
+    app = celery.Celery()
+    app.config_from_object(celery_config(tg.config))
+    return app