Fix ssh-auth.absent to search for just the key; Fixes #4170

Previously was searching for the whole line, and ssh.rm_auth_key and
ssh.check_key were both expecting just the key part.
Set result=None not return=None for test=True
Merge pull request #4550 from basepi/sshfix4170

Fix ssh_auth.absent to check only the key, not the whole key line