Notify user if no database is specified
Notify user if no database is specified

This CR raises a sensible exception if the user has not specified a
proper SQLAlchemy database URL either through a CLI argument or
through the config file.

Change-Id: I769c349681bb2d762d5b47bbaced0b7b780641d7
Closes-Bug: 1328249

cneill/barbican,jmvrbanac/barbican,openstack/barbican,cloudkeep/barbican,MCDong/barbican,openstack/barbican,cneill/barbican,cloudkeep/barbican,MCDong/barbican,jmvrbanac/barbican