We can not use DEVSTACK_GATE_TLSPROXY=1 when building stable/ocata.
Update it to not set it if we have BRANCH_OVERRIDE or the ZUUL_BRANCH
set to stable/ocata.
This will allow us to build stable/ocata from the
openstack/ironic-tempest-plugin gate jobs.
Change-Id: Id487b997a9d83c5daf2f9284a4e80e1f90fe6ce2