Don't use atomic: in test/extended/images/signatures.go
Don't use atomic: in test/extended/images/signatures.go

- Use docker:// instead of atomic:, as recommended back in
  https://github.com/openshift/origin/pull/21782#issuecomment-459077188
  https://github.com/openshift/origin/pull/21782#discussion_r253249564

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

openshift/origin,jsafrane/origin,soltysh/origin,akram/origin,mahak/origin,akram/origin,xiuwang/origin,bparees/origin,sallyom/origin,xiuwang/origin,sallyom/origin,mahak/origin,openshift/origin,JacobTanenbaum/origin,spadgett/origin,JacobTanenbaum/origin,JacobTanenbaum/origin,csrwng/origin,coreydaley/origin,spadgett/origin,coreydaley/origin,mahak/origin,openshift/origin,deads2k/origin,bparees/origin,deads2k/origin,mfojtik/origin,jsafrane/origin,spadgett/origin,soltysh/origin,sallyom/origin,coreydaley/origin,akram/origin,csrwng/origin,gabemontero/origin,jsafrane/origin,xiuwang/origin,bparees/origin,deads2k/origin,soltysh/origin,hroyrh/origin,gabemontero/origin,spadgett/origin,mfojtik/origin,hroyrh/origin,csrwng/origin,mfojtik/origin,hroyrh/origin,spadgett/origin,gabemontero/origin