This was causing "No JSON object could be decoded"

Prevent creating 2 wrapper for the same service if hash changed. Here, one service is exited because port is used by the firt to service to start:

    slappart6:runner-sshd-4248650e36a9a26a6481df1baffd9f58-on-watch                RUNNING   pid 27835, uptime 0:03:45
    slappart6:runner-sshd-b3b68f4278ceb84691ec27521ea229eb-on-watch                EXITED    Mar 06 04:52 PM

To achieve that, update slapos.cookbook and use hash-existing-files option of wrapper recipe

hash-existing-files list all the files used for hash that are not
handled by buildout. For those files, the hash is calculated as soon as
the __init__ function so that if there is a change in those files,
buildout will remove the existing wrapper (it will uninstall the
section) and replace it with the new wrapper.

/reviewed-on nexedi/slapos!525

/reviewed-on nexedi/slapos!172

powerdns: provide basic instanciation

powerdns: add monitoring, promise and reload

powerdns: add replication

power-dns: replication specific for powerdns

powerdns: fix parameter name

powerdns: provide input json schema for powerdns instance

powerdns: Publish current NS record

powerdns: introduce flexible configuration for countries/ geographical zones

powerdns: fix contry configuration list

powerdns: fix country map list

powerdns: add dynamic slave configuration

powerdns: remove recursor

powerdns: reduce log level

powerdns: provide basic instanciation

powerdns: add monitoring, promise and reload

powerdns: add replication

power-dns: replication specific for powerdns

powerdns: fix parameter name

powerdns: provide input json schema for powerdns instance

powerdns: Publish current NS record

powerdns: introduce flexible configuration for countries/ geographical zones

powerdns: fix contry configuration list

powerdns: fix country map list

powerdns: add dynamic slave configuration

powerdns: remove recursor

powerdns: reduce log level