As part of a cleanup effort leading toward modern argument parsing

support, this change moves some of the SiteList parsing/management

code out of main() and into each individual command class. That avoids

the mixed logic we had before, simplifying main(), and enables

testability.
Unit tests were added, as part of an effort to begin building a test

suite for rb-site.

Unit tests pass.
Tested each command with valid and invalid site pathhs.
Tested the upgrade command with --all-sites.