Don't enable the dstat service in CI jobs

It's already disabled for devstack jobs with
[1]. Let's do for grenade too as those too
seeing this issue.

[1] https://review.opendev.org/q/I32ef8038e21c818623db9389588b3c6d3f98dcad

Related-Bug: #1943184
Change-Id: I72db099726ec6f737bc8eccd94a44fd715552728
(cherry picked from commit 79a138f866)

.zuul.yaml

@@ -200,7 +200,7 @@
         # Core services enabled for this branch.
         # This list replaces the test-matrix.
         # Shared services
-        dstat: true
+        dstat: false
         etcd3: true
         mysql: true
         memory_tracker: true
@@ -270,7 +270,7 @@
           # Core services enabled for this branch.
           # This list replaces the test-matrix.
           # Shared services
-          dstat: true
+          dstat: false
           memory_tracker: true
           # TODO: tls-proxy is not tested by legacy jobs;
           # enable this after testing.