By default, `--tags=self-check` no longer validates certificates
when `matrix_ssl_retrieval_method` is set to `self-signed`.

Besides this default, people can also enable/disable validation using the
individual role variables manually.

Fixes #124 (Github Issue)

The Server spec says that redirects should be followed for
`/.well-known/matrix/server`. So we follow them.

The Client-Server specs doesn't mention redirects, so we don't
follow redirects there.