make qonos migration's db_version work similar to nova

- extracted out a new function named get_engine() from configure_db() on
  qonos.db.sqlalchemy.api module for using it for db migration
- made necessary code changes to the migration functions to use
  get_engine() from qonos.db.sqlalchemy.api module.
- removed current_version argument from db_sync() function
- added missing entity models for auto creation of the db that happens
  when qonos-api process is started.
- removed the migration script's direct dependancy on oslo.config.

For: RM#4826