Rework configurable 'PW_TEST_DB_xxx' settings

Rework configurable 'PW_TEST_DB_xxx' settings

There are a number of environment variables that users can set to
configure different aspects of their testing environment. Rework these
like so:

* People use PostgreSQL, so make it as easy as possible for them to
  develop and test against it. Add a 'PW_TEST_DB_TYPE' setting
* Attempt to use defaults for the username and password in settings
* Allow the user to configure the database name, if they so wish

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>