Bug 13791: make koha-common init script aware of plack

Bug 13791: make koha-common init script aware of plack

This patch makes the packages' koha-common script aware of plack.
It does so by relying on koha-list --plack to know which instances
have Plack configured, and uses the koha-plack script to manage
the running daemons.

It also introduces a plack_status function to check the status of
the Plack daemon when called:

   $ servive koha-common status

Regards

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>