core: Move getFQN to SetVmTicketCommand

The method VdcUser.getFQN() is only used in the SetVmTicketCommand. This
patch moves the method to SetVmTicketCommand to simplify the VdcUser
class.

Change-Id: If4a3a45136db9c42ab347f9722326f597afd26fd
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>