Allow to skip the host check when applying or getting a playbook

Otherwise you could not apply a playbook generically and programmatically, i.e. the `configure-jailhost` command from bsdploy was broken by this feature, if it wasn't optional, see https://github.com/tomster/bsdploy/commit/a9c28a170b6b5f14a89810e86fbda814631d567e

also, even if this PR shouldn't be merged, this fixes a bug, if hosts is None, because then line 486 would raise a `Nonetype not iterable` :)