Fixed a bug in default value of $listen_address

The default value of $listen_address was ["0.0.0.0"] but as it was not treated
as an array in the template the resulting sshd_config was broken. Made the
default value '0.0.0.0' (as in class documentation) to fix this issue.

Signed-off-by: Samuli Seppänen <samuli.seppanen@gmail.com>