Refactor UserManager.findIdentityKeyWithProperty to findIdentitiesWithProperty

The method in the class UserManager

public Identity findIdentityKeyWithProperty(String propName, String propValue)

returned an Identity. This is wrong in two ways: 

• it can find multiple identities, but only the first one is returned
• it does not return an identity key as suggested by the method name but a real identity. 

The code is refactored to fix those two minor issues (no end user change, only internal code beautify)

public List<Identity> findIdentitiesWithProperty(String propName, String propValue)