Detect validity of hostname, warn and fallback to IP if invalid

We should provide guidance to users if the hostname of their system is not a valid DNS name. At the end of the gitlab-ctl reconfigure process, we should do a DNS lookup of the hostname. If it fails to resolve, we should provide a warning at the end of the output. We should also utilize the IP address instead of the hostname, since that has a higher chance of working.

Something like `WARNING: This computer's hostname is not a valid DNS entry. Edit "/etc/gitlab/gitlab.rb" and change "externalURL" to a valid URL.

cc @marin, WDYT? I set for milestone 9.5 as it seems like it should be pretty low effort?

changed title from Detect validity of hostname and warn if it is invalid to Detect validity of hostname, warn and fallback to IP if invalid

changed the description

mentioned in issue #2520 (closed)

Closing in favor of: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2620

closed