The latest `dns-lexicon` won't install on that platform.

fixes clutterbox#18

Defaults to `"{{ dehydrated_install_root }}/dehydrated --cron"`

There were subtle variations on the call to this scattered through the
code, and this way one may change the command, or embed into a pipe to
redirect output as needed.  For example:

- 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.