make sure all identifiers in PostgreSQL DBs use safe characters only

[derhuerst]

From #38 (comment):

<entry key="database">park-api</entry>

[…] - seems to be invalid for PG identifiers (including DB names). At least, it doesn't match the convention of "letters, digits, underscores".

Using - doesn't seem to fail, but we should move to the documented intended naming scheme.