Added ssh-cookbook dependency for better sshd_config management

This change uses the ssh-cookbook by @gerhard to manage changes to
sshd_config.

This also implements a change to the attributes AllowTcpForwarding and
PermitTunnel: now they must be either ‘yes’ or ‘no’ in order for sshd
to function properly.
Merge pull request #1 from tcanabarro/master

Add ssh-cookbook dependency for better sshd_config management