Allow to change prometheus scrape annotations

[grzesuav]

In our setup we have different prometheuses, one special for coroot and one for the whole cluster.

I have coroot promehteus targeting specific coroot pods - node agents and ksm - however due to presence of prometheus.io/scrape: 'true' it is also scraped by global fleet, which is undesired.

Currently this annotation is hardcoded, my proposal is to move it to values.podAnnotations - it still be a default, however people coud change it if needed

[grzesuav]

@def implemented this in #28 in backward compatible manner