Fix and refactor launcher (part 1)

### Motivation
Improve the consistency and reduce the verbosity of the launcher; use `argparse` more aggresively. There will be about 3 or 4 PRs, each reducing the launcher size by about 100 lines.

### Changelog
* Fix various bugs and typos.
* Structure the parser arguments in a way that is easy to read and extend.
* Name the parser arguments, the `defaults` keys and the `OpenBazaarContext` atttributes with consistent names. Some flags are removed, others are renamed; consult the `usage` message.
* Reuse the `Namespace` object returned by the parser; it has all the default options built in.
* Remove execute flag from various `*.py` files.