Replace clear_table with more generic init_copy overridable method

Change-Id: Ia65a5029931f37dd98615166e53e4c62f0bd9301
Reviewed-on: https://gerrit.spotify.net/gerrit/46046
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>
Reviewed-by: Elias Freider <freider@spotify.com>

Conflicts:
	luigi/postgres.py