Change mysql CLI call to native PHP function

[wojtekn]

It fixes https://github.com/wp-cli/config-command/issues/152

In this diff, I propose to change the way of validating MySQL connection from the CLI command call to the native PHP function.

$wpdb is not accessible, so I used the native mysqli function, similar to what happens in $wpdb->db_connect().

Testing steps

1. Prepare a directory that contains WordPress code
2. Remove wp-config.php if it exists
3. Try creating a config file using incorrect database credentials

../config-command/vendor/bin/wp config create --dbname=test --dbuser=test --dbpass=wrongpassword --locale=ro_RO --dbhost=127.0.0.1

4. Confirm that the error is shown and mentions incorrect credentials, e.g.:

Error (1045) Access denied for user 'test'@'172.17.0.1' (using password: YES)% 

5. Run the command with the correct database credentials
6. Confirm that the config file was created and success message was displayed:

Success: Generated 'wp-config.php' file.

[kozer]

@wojtekn , can you please provide some test instructions here?