engine: Do not show 'Linux boot options' for OS types - Unassigned, Other

Linux boot options are now visible only for Linux OS types, not for Unassigned
or Other.

Also changed the name of related IsLinux_Unassign_UnknownOS property to
IsLinuxOS since it is used exactly for the purpose of determining whether to
show the Linux Boot Options or not.

Change-Id: I107207f28575f097e2bdf9620bb63d51116a9515
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=909127
Signed-off-by: Martin Betak <mbetak@redhat.com>