Use templated value for Gateway cm name (#4534)

Fixes #4531 

This PR updates the `linkerd-gateway` cm's name to be templated. To allow multiple Gateway installations in the same cluster with different configmaps.

(Installing multiple gateways in the same cluster is possible only through Helm, as the CLI dosen't expose those commands currently.)

Signed-off-by: Tarun Pothulapati <tarunpothulapati@outlook.com>

charts/linkerd2-multicluster/templates/gateway.yaml

@@ -3,7 +3,7 @@
 apiVersion: v1
 kind: ConfigMap
 metadata:
-  name: linkerd-gateway
+  name: {{.Values.gatewayName}}-config
   annotations:
     {{.Values.createdByAnnotation}}: {{default (printf "linkerd/helm %s" .Values.linkerdVersion) .Values.cliVersion}}
   namespace: {{.Values.namespace}}
@@ -64,7 +64,7 @@ spec:
       volumes:
         - name: config
           configMap:
-            name: linkerd-gateway   
+            name: {{.Values.gatewayName}}-config
       containers:
         - name: nginx
           readinessProbe: