prevent duplicated calling of db:schema:load

After adding FKs the fact that dev-tools call db:schema:load twice causes the
install to fail. This is caused by the way rails handle situation when creating
a table that already exist: it calls 'drop table', but that is needed is 'drop
table cascade'.

This is martinpovolny's fix from https://github.com/aeolus-incubator/dev-tools/issues/42 .