Bug 24905: Make installer update log4perl.conf for instances

Bug 24905: Make installer update log4perl.conf for instances

This patch makes koha-common.postinst check if the log4perl.conf files
on the instances need tweaking. If they do, it appends the relevant
configuration text.

To test:
1. Install Koha 19.11 or earlier using the packages
2. Create an instance:
   $ koha-create --create-db test1
   $ koha-create --create-db test2
=> FAIL: /etc/koha/sites/test1/log4perl.conf doesn't include an entry for
z39.50
3. Apply this patch and run:
   $ docker run \
      --volume=/path/to/your/clone:/koha \
      --volume=/dest/path/for/debs:/debs \
      --privileged
      --env VERSION=19.12.00.047 \
      koha/koha-dpkg:master
Note: Paths need to be adjusted
4. Grab the .deb files and put them on the machine you're testing this
5. Edit /etc/koha/sites/test2/log4perl.conf and add the following text
   at the bottom: log4perl.logger.z3950
6. Install the debs:
   $ sudo dpkg -i koha-perldeps koha-deps koha-common
=> SUCCESS: Install process doesn't explode due to the addition
=> SUCCESS: /etc/koha/sites/test1/log4perl.conf has a new entry for log4perl.logger.z3950
=> SUCCESS: /etc/koha/sites/test2/log4perl.conf doesn't have a new entry for log4perl.logger.z3950
7. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>