Make ssh timeout configurable

[mark-dce]

We've hardcoded the ssh timeout into our config and may have cases (like building VMs or training EC2 instances for Camps) where we either want longer or no timeouts.

Possible options would be to make the timeouts settings configurable variables, or to pull out the timeout setting and make it a separate role:

  roles:
    ...
    - { role: sshd_config,  ClientAliveInterval=0 }
    ...

OR

  roles:
    ...
    - { role: sshd_config }
    - { role: sshd_timeout }
    ...