[FS-266] Fix SFT URL fecthing based on plain HTTP via IPv4

[mdimjasevic]

This is part of https://wearezeta.atlassian.net/browse/FS-266:

• It allows for using a plain HTTP connection given an IPv4 address of an SFT server,
• It adds a proper integration test for sft_servers_all as the previously extended test silently ignored a code path.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[jschaul]

The existing code introduced in #2012 #2014 #2015 seems to be broken, so this PR may improve on the status quo.