vmagent: create shard sevices when using vmagent cluster

[Haleygo]

When enabling -promscrape.cluster in vmagent, it's impossible to know which shard we're using through the general service.
It would be helpful to have a seperate svc created for each shard vmagent deployment.

[Haleygo]

Also related to #604

[f41gh7]

I don't think that it's useful to have separate service per each shard.

It's already possible to access each shard pod with headless service by corresponding pod name. It requires statefulMode: true.

[f41gh7]

Closing since proposed changes doesn't look useful.