tests/engine_test:

Fixed a FK relationship in a test fixture to prevent spurious
crashes when running bin/openquake --lhc.
tests/engine_test:

Removed a useless test case.
save memory by saving reference to sites objs only once
Merge branch 'misc-fixes' of https://github.com/larsbutler/oq-engine into risk-from-ses
added migration sql scripts to disttools package