Remove ZUUL_CHANGE

Removing old ZUUL_ env variables which have been replaced with
Ansible variables.

While we're at it, ensure that zuul.change and zuul.patchset are
both rendered to Ansible as strings so that we don't build any
unecessary assumptions about the type of those values.

Change-Id: I3de53c58ac2c5a35547e22a79b8b42558610284a
3 files changed