Set the earliest-version tag for each remaining releasenotes directive
to avoid having to scan back to the first version of nova.
```
$ git clean -xfd; tox -e releasenotes --notest; \
time tox -e releasenotes
```
Without this change:
```
real 0m25.606s
user 1m20.401s
sys 0m5.063s
```
With this change:
```
real 0m22.690s
user 1m5.723s
sys 0m4.369s
```
Change-Id: Id6d318c7502319e9206bd03296b15a02454622c7
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
Stephen Finucane
2ce813bf03