Gracefully handle custom VS and TS template change in ConfigMaps

[jjngx]

Is your feature request related to a problem? Please describe.

This is a follow up of this PR focusing on Ingress and Main template lifecycle.

Describe the solution you'd like

• NIC reverts to using default VS and TS templates without the need for restarting the pod.

• When users delete their custom VS or/and TS templates defined in a ConfigMap, NIC reverts to using default TC and VS templates.

Describe alternatives you've considered

N/A

Additional context

More info about the context here.