Refactor the ManagementUtility to make get_commands a method

[d9pouces]

The ManagementUtility directly uses the get_commands function instead of encapsulating it in a method that could be overridden, for example to exclude some unwanted Django commands.

To ensure compatibility, the simplest way is to add a get_management_commands method to ManagementUtility that only calls the original get_commands function.

[Mariusz Felisiak]

I don't see any reason to exclude management commands, or at least this is not something that would be widely used. Please first start a discussion on the DevelopersMailingList, where you'll reach a wider audience and see what other think, and ​follow the guidelines with regards to requesting features.