Add https support to apache2 on ICF-CI

[christian-monch]

This PR enables SSL on the test-HTTP server. It installs all necessary certificates to enable requests and git-annex to establish connections to the server.

The PR changes the data_webserver-fixture to return an https:-URL on an appveyor-VM. So all tests should run against an https:-endpoint.

Fixes #17

BEFORE this PR can be merged, the following tasks have to be completed:

• [ ] merge #36
• [ ] modify ./singularity/icf.def:27 according to the comment after #36 was merged.
• [ ] remove the certificates in .appveyor/data from this branch (optional)

To indicate that the previous tasks should be completed before the merge, this PR is put into draft mode

[christian-monch]

The remaining errors are not related to https (datalad get fails)

[christian-monch]

Remaining get-errors where fixed by merging the latest changes from main into the branch.