Nosetests fail in test_databases, when bob.db databases are not available

Created by: siebenkopf

When setting up a new package as described here: http://pythonhosted.org/bob.bio.base/installation.html#installation-instructions adding the bob.bio.face package, but omitting some or all of the bob.db database interfaces, the nose tests will not pass on the omitted databases. An ImportError is raised, when the database configuration is read. We should have a mechanism that avoids raising these import errors, at least while running the nose tests.

Also, the omitted databases are still listed in the ./bin/resources.py, while they are not really ready to be used.