provide a healthcheck option

[evgeni]

In #352 it came up that it'd be cool if foremanctl would be able to perform health checks

• as the last step of the deployment
• as a standalone command

[ehelms]

I think our general, past assumption, is that if the installation succeeds, then that is the "check". However, I think we've learned there can be some nuance to that and having some checks at the end once everything comes together could be useful to help ensure the integrity of the install meets expectations.