A new configuration *createiso_use_xorrisofs* is added to determine
which tool to use for creating ISOs.
By default, createiso_use_xorrisofs = False and genisoimage will be used.
When set to True, xorrisofs will be used.
JIRA: RHELCMP-2875
Fixes: https://pagure.io/pungi/issue/1130
Signed-off-by: Haibo Lin <hlin@redhat.com>