Bug 13359 - allow the packages to use virtual-mysql-*

Bug 13359 - allow the packages to use virtual-mysql-*

Prior to this patch, the koha packages depended on mysql-client. This
meant that it was tricky to use things such as the MariaDB libraries
instead.

This patch recommends mysql, but will accept mariadb and other things
that are marked by debian as able to replace mysql.

Signed-off-by: wajasu <matted@34813.mypacks.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit 9419177a1e39296fe1049b44fb9788eeb4341b77)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>