allow custom go-md2man binary
allow custom go-md2man binary

Signed-off-by: Lokesh Mandvekar <46b23e80209bb43857d25fd87bafe9be89c7d33a@redhat.com>

kubernetes-incubator/cri-o,nalind/cri-o,kubernetes-incubator/ocid,kubernetes-incubator/cri-o,nalind/cri-o,kubernetes-incubator/ocid,kubernetes-incubator/cri-o,mikebrow/cri-o,mikebrow/cri-o,kubernetes-incubator/ocid,mikebrow/cri-o,nalind/cri-o,kubernetes-incubator/cri-o,mikebrow/cri-o