base: database script documentation formatting

* Makes the documentation in scripts/database.py follow PEP257 rules.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
base: debug improvements for load_fixtures

* Throws an exception when there are fixtures
  without matching models.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
base: single fixture classes loading

* Makes possible to load single fixture classes.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>