meshConfig: add validation for ingressgateway certificate (openservic…

…emesh#4422)

Signed-off-by: Sneha Chhabria <snchh@microsoft.com>

Validation added to the subjectAltName for the ingress gateway
certificate. Going forward, this field if specified will have to contain
atleast one element in the array.

Fixes openservicemesh#4353

cmd/osm-bootstrap/crds/config_meshconfig.yaml

@@ -194,6 +194,7 @@ spec:
                           type: array
                           items:
                             type: string
+                          minItems: 1
                         validityDuration:
                           description: Certificate validity duration, represented as a sequence of decimal numbers each with optional fraction and a unit suffix
                           type: string