Support --env-db-password=DB_PASSWORD

[adamziel]

Including database credentials in the CLI command is risky in that anyone with ps access may read them. Let's support import.php CLI flags to point the script to specific ENV variables holding the database username, host, password, etc. They would be alternatives to --db-* flags, so there would be --db-password=password and an alternative flag --env-db-password=DB_PASSWORD that would source the password from the DB_PASSWORD env variable.