Geo: Test sshd is working correctly before running QA tests

Problem

In https://gitlab.com/gitlab-org/quality/nightly/issues/123#note_203866724 it was discovered that sshd was not working correctly (well, gitlab-shell to be precise) but it took quite a while to determine this was the core of the issue.

Proposal

Similiar to how #wait_for_services works which tests the web side of thinks is OK, we should add a basic test to ensure sshd is working as expected.