bifrost-prepare-for-test-dynamic: Create known_hosts if it's not present
Create an empty ~/.ssh/known_hosts if it's not present since the subsequent
commands assume that there is one in place. Fixes the following problem:
fatal: [testvm1]: FAILED! => {"changed": true, "cmd":
["ssh-keygen", "-R", "192.168.122.3"], "delta": "0:00:00.004327", "end":
"2016-09-14 08:26:42.448637", "failed": true, "invocation": {"module_args":
{"_raw_params": "ssh-
keygen -R \"192.168.122.3\"", "_uses_shell": false, "chdir": null, "creates":
null, "executable": null, "removes": null, "warn": true}, "module_name":
"command"}, "rc": 255, "start": "2016-09-14 08:26:42.444310", "stderr":
"ssh-keygen: $
root/.ssh/known_hosts: No such file or directory", "stdout": "",
"stdout_lines": [], "warnings": []}
Change-Id: I0f05fb9aad7e7b9021491aac4d809ffc6fd4e864
7b005fd1ec