SSH ForceCommand is now optional

When sftp_force_command is false, we can force a per user specific
command by putting the "command" option in the .ssh/authorized_keys for
the user in question.

Example:
/home/user/.ssh/authorized_keys
command="rsync --server -av --delete . /my/dir", ssh-rsa AAAA....

templates/sshd_config.j2

@@ -6,7 +6,9 @@ Match Group {{ sftp_group_name }}
     AllowTCPForwarding no
     AllowAgentForwarding no
     X11Forwarding no
+{%if sftp_force_command %}
     ForceCommand {{ sftp_force_command }}
+{% endif %}
     PubkeyAuthentication yes
     PasswordAuthentication {% if sftp_allow_passwords %}yes{% else %}no{% endif %}