Fix install on Debian Docker images (fix runlevel problem #479)

An alternate approach to fixing the runlevel problem https://github.com/saltstack/salt-bootstrap/issues/479 as compared to PR https://github.com/saltstack/salt-bootstrap/pull/480 (though that also looks like it works).

In this approach:

- No changes for most users
- In Debian Docker images (and perhaps other environments) when `runlevel` == "unknown":
  - DO NOT check that the services will start at boot.
  - DO NOT exit with a fatal error caused by not being able to check that the services will start at the current runlevel.

In Docker the startup scripts don't get run so this is a pointless check in that event anyway, and without this fix Salt cannot be installed.