Bug 18709: Make koha-foreach use koha-shell internally

Bug 18709: Make koha-foreach use koha-shell internally

This patch simplifies the koha-foreach script, making it use koha-shell
to gain instance privileges and have all environment variables set.

To test:
- Apply the patch
- Have one or more instances created (kohadev already exists in kohadevbox, add
  another onewith $ sudo koha-create --create-db test)
- Run:
  $ sudo kohaclone/debian/scripts/koha-foreach echo "This is instance __instancename__"
=> SUCCESS: The script runs gracefuly, and outputs something like:
This is instance kohadev
This is instance test
  Note: this means the command was executed for each instance and the current behaviour
        of replacing the placeholder __instancename__ for the actual instance name still
        works as expected.
- You can try the different filter switches, but the logic has not been touched. only the
  command call.
- Sign off :-D

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>