Rename test databases when running parallel Tox

When tests are executed using Tox in parallel, modify the test database
names, to avoid name collisions between processes.

This change renames the existing
`django_db_modify_db_settings_xdist_suffix` fixture, to a generic
`django_db_modify_db_settings_parallel_suffix` one, in case more
scenarios/tools have to be considered in the future.

It also handles projects where both `pytest-xdist` and parallel `tox`
are being using, generating database names like
`test_default_py37-django21_gw0`.

Resolves #678.

Author: adamantike