CI: Disable heat in upgrade jobs to save disk space

We see frequent failures of upgrade jobs on stable branches due to lack
of disk space. Disable heat in an attempt to free up extra space, since
we do not need it for our testing.

Change-Id: I20c99ca9dd6e78b041e4662c9b1b54eac904e3c9

playbooks/kayobe-overcloud-upgrade-base/overrides.yml.j2

@@ -39,3 +39,6 @@ aio_bridge_ports:
 
 # Enable ironic for testing baremetal compute.
 kolla_enable_ironic: true
+
+# Disable heat to save disk space.
+kolla_enable_heat: false