Fix changing the password when the password manager is not available

Leaving the state in NoKeychainBackend would break any further attempt
to get the password because CredentialStore::canTryAgain would return
false.
Credidentail dialog: it was suposed to be asynchronous
Fix reading the password from the config

The code was expecting pwdba to be in base64, which it was not if it
came from the "password" field