db: better support for databases with "odd" characters in the name, such as "-"

Add missing quoting, using the quoting flavour preferred by the DB.

Tested with

echo "CREATE USER 'kallithea-test'@'localhost' IDENTIFIED BY 'password'"|sudo -u mysql mysql
echo "GRANT ALL PRIVILEGES ON \`kallithea-test\`.* TO 'kallithea-test'@'localhost'" | sudo -u mysql mysql
TEST_DB='mysql://kallithea-test:password@localhost/kallithea-test?charset=utf8mb4' py.test

sudo -u postgres createuser 'kallithea-test' --pwprompt --createdb
TEST_DB='postgresql://kallithea-test:password@localhost/kallithea-test' py.test