Bug 1020301: Fix mariadb-server-10.3 postinst issue

Currently any plugin installation causes error:

• postinst called with unknown argument 'triggered'

This is because debian/mariadb-server-10.3.postinst does not have upstream added 'triggered' target.

Pipeline won't go cleanly without MR !32 (closed)

These are compination of upstream commits (made for 10.2): f25a5c39c1467df4677663607625c7444e65f837 and 82a2ea64b47ba7d4f10019418f84a0f3131d1c11