Turn Kerberos-related properties to Service class members
The Service class now accepts keytab path and service name part of Kerberos
principal as members. Kerberos principal is turned into a property computed
from service prefix, FQDN and realm. the handling of Kerberos principals and
keytabs in service installers was changed to use class members instead of
copy-pasted constants. This shall aid in the future refactoring of
principal/keytab handling code.
Reviewed-By: Stanislav Laznicka <firstname.lastname@example.org>