Allow user/playbook to specify dehyraded cron call

[eengstrom]

I had need to change the call made to dehydrated as embedded into the cron or systemd service. Also, there are subtle variations on the call to this scattered through the code. With the proposed change, one may change the command, adding arguments or even embed into a pipe to redirect output as needed.

Documentation and defaults added. Default value for new variable:

dehydrated_renew_command: "{{ dehydrated_install_root }}/dehydrated --cron"

Alternative example uses:

• Don't give up on first failure:

dehydrated_renew_command: "{{ dehydrated_install_root }}/dehydrated --cron --keep-going"

• More complicated example to redirect output to syslog:

dehydrated_renew_command: "bash -c '({{ dehydrated_install_root
}}/dehydrated -c -g | logger -t dehydrated -p local7.info) 2>&1 | logger
-t dehydrated -p local7.error -s'"

... although the latter is somewhat contrived and one would probably be
better served by using a systemd timer rather than a cron job, which
would then do effectively the same thing.