sshd: allow setting a null ssh config

sshd: allow setting a null ssh config

The same effect could already be obtained if the ~/.ssh/config file
did not exist. But that is more difficult to control by clients,
since JGit would pick up the config if it was then created. Therefore
allow specifying a null config explicitly to permanently switch off
config file handling.

Change-Id: Iedf8a7f4d5c1ca08e0a513ed28301d8e5261b22a
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>