Pass vars to logging.config.fileConfig

This allows one to set up a logging configuration that is parameterized
based on variables specified on the command-line.

e.g.: the application .ini file could have:

```ini
[logger_root]
level = %(LOGGING_LOGGER_ROOT_LEVEL)s
handlers = console

[handler_console]
class = StreamHandler
args = (sys.stderr,)
level = %(LOGGING_HANDLER_CONSOLE_LEVEL)s
formatter = generic
```

This app could be launched with:

```
pserve development.ini LOGGING_LOGGER_ROOT_LEVEL=DEBUG LOGGING_HANDLER_CONSOLE_LEVEL=DEBUG
```

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