scep client: support externally defined passwords via encrypted config entry

[lefou]

The current values for blended.security.ssl.storePass and blended.security.ssl.keyPass are used as seeds to internally generate the real password. This design doesn't allow the usage of externally defined passwords. Hence, apps using the scep client are forced to use the generated password from the scep client.

Solution: The app or some external entity should give the passwords to us. Of course, config files needs to support the encryption syntax (#91). The feature to generating passwords from seed can be kept as a fallback.