Honor $WEB_CONCURRENCY environment variable.

Allows for changing concurrency configuration without changing
application code. This is an environment variable that other
communities (e.g. Rails) are starting to follow, and have enjoyed for a
little while now.

Have discussed with /cc @benoitc in the past and he was +1

gunicorn/config.py

@@ -98,7 +98,8 @@ class Config(object):
 
     @property
     def workers(self):
-        return self.settings['workers'].get()
+        env_workers = int(os.environ.get('WEB_CONCURRENCY'))
+        return self.settings['workers'].get() or env_workers
 
     @property
     def address(self):