Start cleaning up global install and adding tests

See commit messages for details. But basically replace git-clone stuff with go-install, make the role idempotent (at least when running only the primary), and add actual molecule tests (primary only so far). Few other minor clean-ups but nothing major.

One breaking change: there's no longer a sigsum_configdir_base. So if any user set this to something different than the same value which is now hardcoded with the same default value -> unexpected behavior like generating a new log key.

One somewhat breaking change: users that expect the git-clone directories to be updated will be surprised that they get the right go tools for the specified versions while the git-clone directories stay at the same old versions (since we no longer go-install from the source directories).

My thinking: we are only aware of one user (us), so no-one will be bothered by these changes. And we can signal that there's been interesting changes by going from current v1 tag to v2 tag for the collection. There are more clean-up / fixes we need to do which will be breaking.