flex: enable flex driver in the integration tests

The flex agent was mistakenly disabled in the CSI config change
in this PR: https://github.com/rook/rook/pull/5032. The agent must
be enabled for integration tests.

Signed-off-by: Travis Nielsen <tnielsen@redhat.com>

tests/framework/installer/ceph_manifests.go

@@ -1498,7 +1498,7 @@ spec:
             fieldRef:
               fieldPath: metadata.namespace
         - name: ROOK_ENABLE_FLEX_DRIVER
-          value: "false"
+          value: "true"
         - name: ROOK_CURRENT_NAMESPACE_ONLY
           value: "false"
 ---