Allow use of no kickstart for systems

Before, if a system had an empty string as kickstart then it would
inherit the one from its profile. It is not desirable if we don't want to use
a kickstart at all, as is the case of attended installations.

Signed-off-by: Alan Evangelista <alanoe@linux.vnet.ibm.com>
Signed-off-by: Andrea Bucci <andreavb@linux.vnet.ibm.com>
Signed-off-by: Eduardo Bacchi Kienetz <ebacchi@linux.vnet.ibm.com>
Signed-off-by: Plinio Freire <pleirie@linux.vnet.ibm.com>
Merge pull request #669 from natoze/support_no_system_kickstart

Allow use of no kickstart for systems