[64143110] Fix path for Icinga p1.pl
[64143110] Fix path for Icinga p1.pl

This was preventing the Icinga service from starting. Worked fine on existing
machines because Nagios hadn't yet been uninstalled and it could use the
old file.
```
Jan 30 16:45:57 monitoring-1 icinga: Error: p1.pl file required for embedded Perl interpreter is missing!
Jan 30 16:45:57 monitoring-1 icinga: Bailing out due to errors encountered while initializing the embedded Perl interpreter. (PID=30699)
```

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet