This fixes a regression since the change done in c1c152f7ac.
When another role (say `matrix-jitsi`) included `roles/custom/matrix-base/tasks/util/ensure_openssl_installed.yml`,
which then included `{{ role_path }}/tasks/util/ensure_openssl_installed_DISTRO.yml`,
that `role_path` variable would end up being the parent role
(`matrix-jitsi`) and not the `matrix-base` role, so we'd get a failure.

An alternative solution may have been to avoid using `role_path`, but
importing roles properly (like we've done in this patch) sounds like a better way.

Unfortunately, `import_role` fails if `tasks_from` is something like
`util/ensure_openssl_installed` (containing a `/`), so I had to move
these utils out of `util/`.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2228