Clean up database scripts (create_db.py,db_shell.py,manage_db.py).

Move duplicated code into lib/galaxy/model/orm/scripts.py - add unit tests. PEP-8 clean up of all 4 files. Add incoherent incoherent comment at top of manage_db.py.

As a result of the code de-duplication create_db.py should be usable with the tool shed now.

Will be refactoring lib/galaxy/model/orm/scripts.py to work with new tool shed install database - it will be good to have a place to test these and allow manage_db.py and clean_db.py to work immediately.