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>