Refactor system-wide known_hosts support

System wide SSH fingerprint file will be now generated directly instead
of assembled from separate files. This makes the feature more
obfuscated, previously scanned hosts could be inferred from the
filenames of separate parts.

Changes inspired by:
https://juriansluiman.nl/article/151/managing-ssh-known-hosts-with-ansible
Merge pull request #6 from drybjed/refactor-known_hosts

Refactor system-wide known_hosts support