Make diffie-hellman moduli removal idempotent

[ELLIOTTCABLE]

This change to the shell snippet simply:

1. intentionally leaves a copy of the old version laying around (which subsequent Ansible invocations can check for, to see if this has already been executed); and
2. uses the output from cmp to determine whether any change was actually made to the file.

This gets rid of the 'changed: 1 task' notification when re-running the playbook on a host that's finished setting itself up.

[moltenbit]

Thanks for the PR!