leave the unreleased branch unspecified for release notes
leave the unreleased branch unspecified for release notes

Do not specify a branch for unreleased.rst so it tests notes under
review rather than notes already merged.

Change-Id: If3e4982bcc457906eeebb99de660f6c594b122f0
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

j-griffith/cinder,phenoxim/cinder,Datera/cinder,eharney/cinder,mahak/cinder,phenoxim/cinder,eharney/cinder,Datera/cinder,ge0rgi/cinder,j-griffith/cinder,openstack/cinder,openstack/cinder,mahak/cinder