Disabling c-backup service for osc-functional-devstack-tips job

Since swift isn't compatible with py3 currently and disabled for
this gate job, c-backup service will always fail configuring.
The backup related tests can be handled by other jobs having swift
enabled.
The c-backup service can be enabled along with swift services once
swift is compatible with py3.
This patch disables the the c-backup service for osc-functional-devstack-tips
gate job.

Change-Id: Ifd3a4e1a15f1365107a2a1367513e2ef79bd13cc

Author: rajathere

.zuul.yaml

@@ -166,6 +166,11 @@
         s-container: false
         s-object: false
         s-proxy: false
+        # As swift is not available for this job, c-backup service won't be functional.
+        # The backup related tests can be handled by other jobs having swift enabled.
+        # The backup service along with swift services can be enabled once swift is
+        # compatible with py3
+        c-backup: false
       tox_envlist: functional
       tox_install_siblings: true