sshd_config syntax guard is missing on PermitTunnel

[rvodden]

if a value for ssh_permit_tunnel is passed to the devsec.hardening.ssh_hardening role, then at present that value is passed on without validation.

This is because the value of ssh_permit_tunnel is not guarded in opensshd.conf.j2:

PermitTunnel {{ ssh_permit_tunnel }}

this stands in contrast to other boolean values which have appropriate guards to ensure valid syntax:
e,g,

AllowAgentForwarding {{ 'yes' if (ssh_allow_agent_forwarding|bool) else 'no' }}

This PR resolves this issue and ensures that PermitTunnel will render correctly