Refactor type conversion

The way each opt subclass has to override _get_from_config_parser() just
to perform type conversion is lame. Add an explicit type conversion
callable attribute on Opt and pass it to MultiConfigParser when we're
looking up a value.

This will be useful in a future patch where we'll use MultiConfigParser
(renamed to Namespace) to look up CLI values too - since these get
converted by argparse, we want to avoid converting those values again.

Change-Id: I52003026e788e83c355af55accb73d02150a6b0b
Revert additions to ConfigParser public APIs

In commit 2e1c685, we fixed the MultiConfigParser breakage but we also
added to the public APIs of MultiConfigParser and ConfigParser APIs.

There's no reason to lock ourselves into supporting these new semantics,
so let's add them in a way that they're private to cfg.

Change-Id: Ief1e491590f022dc16cca78b2c4aa2d418aec68f