Fix password generation when adding Admin from Install CLI.

Cherrypicked from croogo/croogo@a04afd1ffa44c6.

The issue seem to be caused by 34906ced29.  Since the dynamic AppModel is
removed from the class registry, the actual Users.User is loaded causing
double hashing on the password in User::beforeSave()
Renaming fixtures to avoid duplicate names
Remove invalid test
Activate Install plugin for testing when debug > 0
Test that user's password is not doubly hashed
Merge branch '1.5-install-fix'