Tests for SSH connect to machine and verify installed OS

[Gerrit91]

One very useful test suite extension would be to connect to the machine through SSH after allocation and do some basic checking on the installed OS (contract verification: OS is corrrect, network config as expected from metal-api data, expected services / processes running, file system layout as described, SSH connect possible, metal user present, ...).

[majst01]

We should then use goss to test some expectation in the actual running image, like frr is running etc.