Update k8s_plugins config format

Previously default.yml file had key-value pairs for the enabled
plugins, for example "intel-gpu-plugin: intelgpu=enabled".
Now simplified to just add a plugins name as a list.

Other related changes are done in below MR:
https://review.opendev.org/c/starlingx/config/+/878509

TEST PLAN:
PASS: bootstrap is success after enabling the k8s plugins (intelgpu)
      in default.yml
PASS: verified “intelgpu: enabled" using
      "kubectl get nodes controller-0 --show-labels" command, after
      enabling the intelgpu k8s plugin in default.yml
PASS: checked whether daemonset pods are running or not, using command
      "kubectl get ds -A | grep 'gpu-plugin'", after enabling the
      intelgpu k8s plugin in default.yml
Note: No GPU hardware is available, so all testing is done using
      commands after bypassing the driver related checks in the code.

Story: 2010604
Task: 48096

Signed-off-by: Md Irshad Sheikh <mdirshad.sheikh@windriver.com>
Change-Id: I1b73dcb9c3b003da27b7389a104c905167830c42

playbookconfig/src/playbooks/host_vars/bootstrap/default.yml

@@ -449,13 +449,13 @@ enable_volume_snapshot_support: false
 # plugins are disabled.
 #
 # "k8s_plugins" is a list of selected vendor device plugins to be
-# enabled as part of the bootstrap. When a device plugin is enabled,
+# available as part of the bootstrap. When a device plugin is enabled,
 # its pods will be deployed when the plugin label (e.g. intelgpu=enabled)
 # is assigned to the hosts in the cluster as part of system bring-up.
 
 k8s_plugins:
-#  intel-gpu-plugin: intelgpu=enabled
-#  intel-qat-plugin: intelqat=enabled
+#   - intelgpu
+#   - intelqat
 
 # SYSTEM APPLICATIONS
 # ===================