ipatests: refactor test for login using cifs alias principal
    
    The test had two problems:
    * if it was failing,  samba services were not started and all other
    tests also failed
    * Utility for copying keys obscured fatal problems i.e. if file does not
    exist or can not be parsed.
    
    Fixed by moving the check to separate test and raising exceptions in
    KerberosKeyCopier on any unexpected problem.
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>