Fix AWS AppMesh issue when providing multiple backends

[jddcarreira]

Signed-off-by: Joao Carreira jddcarreira@gmail.com

When adding multiple backend (by arn or by virtual service name), while using AWS App Mesh as a provided, Flagger was returning the following error:

reconcileVirtualNode failed: VirtualNode test-app-primary update error admission webhook "vvirtualnode.appmesh.k8s.aws" denied the request: VirtualNode-test-app-primary has duplicate VirtualServiceReferenceARNs arn:aws:appmesh:eu-west-1:0000000000:mesh/mymesh/virtualService/api-bar.local

This was caused by a wrong usage of range, since in a range when targeting the pointer of an element it seems to return the object of the slice and not the object of the element in the slice.
https://golang.org/doc/effective_go#slices

Here's a representation of what was happening:

• https://play.golang.org/p/OUdzwkFjEYZ
• https://www.ardanlabs.com/blog/2013/09/iterating-over-slices-in-go.html

In this MR we make use of the index to access the element of the slice, representing each backend.

[stefanprodan]

LGTM

Thanks @jddcarreira