Allow User to Custom the Naming convention "{app_name}-{stage}-{func_name}"

I found there are many ``"%s-%s-%s" % (app_name, stage, func_name)`` in the code base. In my organization they force to use ``{stage}-{app_name}-{func_name}``. Is it possible to create a package level CONSTANT variable in ``chalice/app.py`` like ``NAMING_CONVENTION = "{app_name}-{stage}-{func_name}"`` that allow user to monkey patch this using their own naming convention?

I found five occurrence of this code snippet, I think the refact is as easy as adding at most five variable and fix the import. If the community believe it is a good idea to allow user to change the naming convention, I can issue an PR. Thank you.

Start in chalice/app.py and search the codebase for the five occurrences of "%s-%s-%s" % (app_name, stage, func_name). Check how these names are used before deciding how a package-level convention can be overridden; done means the relevant naming sites use the configured convention without changing existing defaults.

Written by the indexing model from the issue text.