BIOS tests should have ssh key configurable

When running bios tests, i am getting this error:
<status status="FAIL" starttime="20191028 16:20:46.770" endtime="20191028 16:20:46.770" critical="yes">Parent suite setup failed:
Given key file '/root/.ssh/mm747b_id_rsa' does not exist.</status>

Seems that it's generating the key with /root/.ssh/${USERNAME}_id_rsa . While other tests allow to use ${SSH_KEYFILE}, and configure that in variables.yaml . I think that the approach of accepting a KEYFILE is better, as it allows to have keys with a different file name pattern.