keepdb results in "Using existing database" log line, even when no database exists

[Valentijn Scholten]

The log output for running tests with --keepdb can be confusing:

• make sure no test database exists
• run a test with --keepdb
• observe log output Using existing test database for alias 'default'...
• observe that a new test database is created now, without any log output (unless there are migrations which output logs)

Offending code: ​https://github.com/django/django/blob/ca9872905559026af82000e46cde6f7dedc897b6/django/db/backends/base/creation.py#L44

Solution:
Update log lines to reflect what's happening.