The 'Upgrade impact' subsection has been added in the spec template
in I1798f2bdb246af9b9b12dbe9276d0a68a82e4c88.
But 'py27' test fails since it is merged
because the 'Upgrade impact' subsection is required
in existing queens and backlog specs.
So set the 'Upgrade impact' subsection as optional
in queens and backlog specs.
Change-Id: I8c69369a421454f30675a9ad9da8b9c51a2e7baa
Closes-Bug: #1727602
44334e7114