added default value to helm chart values for customPricing.costModel.discount

[sof2000]

Closes: #380
Added default helm value for "customPricing.costModel.discount" at charts/opencost/values.yaml.

Application has logic that use this value as AWS discount which can be used to adjust k8s resources cost. It's just not exposed in values.yaml.

	c, err := aws.Config.GetCustomPricingData()
	if err != nil {
		return nil, err
	}
	if c.Discount == "" {
		c.Discount = "0%"
	}
	if c.NegotiatedDiscount == "" {
		c.NegotiatedDiscount = "0%"
	}

	return c, nil
}

Application code link: https://github.com/opencost/opencost/blob/e977d842ed2ae977726f29637061df0bac352ac6/pkg/cloud/aws/provider.go#L483

Helm values link:

opencost-helm-chart/charts/opencost/values.yaml

Line 540 in 4242fee

costModel:

Values example:

customPricing:
    costModel:
      discount: ""