cubicweb #549136 db-init does not ask db passwd if not set in source file [resolved]

while db-create prompts the user and asks for it if unset hence, on instance creation one fails at db-init time (after being asked the password twice !)

done in3.5.10
load left0.000
closed by#56a8b14d68aa properly fail if user has entered wrong connection information for the system database, closes #549136