feat(MeshGatewayInstance): respect kuma.io/mesh label

[michaelbeaumont]

This is more in line with the plans for targetRef policies.

We need to decide on whether to move to labels everywhere (i.e. Pods/Namespaces)!

Checklist prior to review

• Link to docs PR or issue -- Make clear when to use kuma.io/mesh label or annotation kuma-website#1160
• Link to UI issue or PR --
• Is the issue worked on linked? -- none
• The PR does not hardcode values that might break projects that depend on kuma (e.g. "kumahq" as a image registry) --
• The PR will work for both Linux and Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Unit Tests --
• E2E Tests -- none
• Manual Universal Tests -- none
• Manual Kubernetes Tests -- none
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? --
• Do you need to explicitly set a > Changelog: entry here or add a ci/ label to run fewer/more tests?

[lahabana]

What's the story here @michaelbeaumont ?

[michaelbeaumont]

No story really 😃, just a PR to make MeshGatewayInstance more like new policies