Disable GLOBAL_VENV

Devstack recently introduced the functionality to use a single venv[1]
but some devstack plugins in telemetry project are not compatible with
this change.

This disables the option as a short term workaround.

[1] https://review.opendev.org/c/openstack/devstack/+/558930

Related-Bug: 2031639

Change-Id: I62c31bb942465d3179b906c1eb657272c76e4f23

.zuul.yaml

@@ -46,8 +46,14 @@
             voting: false
         - telemetry-dsvm-integration:
             irrelevant-files: *ceilometer-irrelevant-files
+            vars:
+              devstack_localrc:
+                GLOBAL_VENV: False
         - telemetry-dsvm-integration-ipv6-only:
             irrelevant-files: *ceilometer-irrelevant-files
+            vars:
+              devstack_localrc:
+                GLOBAL_VENV: False
     gate:
       jobs:
         - grenade-ceilometer
@@ -59,5 +65,11 @@
             voting: false
         - telemetry-dsvm-integration:
             irrelevant-files: *ceilometer-irrelevant-files
+            vars:
+              devstack_localrc:
+                GLOBAL_VENV: False
         - telemetry-dsvm-integration-ipv6-only:
             irrelevant-files: *ceilometer-irrelevant-files
+            vars:
+              devstack_localrc:
+                GLOBAL_VENV: False