show plugin commands in separate block
api: add new public Command.usage & Command.summary API
cleanup: make Commands use the new api
test: use Command.usage & Command.summary API in unittest
cleanup: getSummary() and getUsage() can be dropped entirely now.

And a couple of other tweaks related to 0a6051b.
test: fix UsageTest test, so it work without dnf is installed on the system
PEP8 cleanup
fix: provides are not recognized for erase command. (RhBug:1087063)