shadow pod group derives min member from pod annotations

[mateuszlitwin]

What this PR does / why we need it:
Added scheduling.k8s.io/group-min-member annotation that allows to specify min member for the shadow pod group.

Requiring users to learn new custom resource PodGroup and properly handling clean up of such resource is nontrivial and time consuming. In common use case user just want create a batch of pods that will be allocated together at the same time. Extending shadow pod group capabilities could be a solution for this.

Which issue(s) this PR fixes:
Fixes #905

Special notes for your reviewer:

Release note:

Shadow pod group has min member derived from the pod annotation "scheduling.k8s.io/group-min-member". This is backward compatible change - if annotation is not specified, then min member defaults to 1. If pod has normal pod group, then this annotation will be ignored, and min member from the pod group object will be used instead.

[k8s-ci-robot]

Hi @mateuszlitwin. Thanks for your PR.

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[mateuszlitwin]

/assign @k82cn

[k82cn]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: k82cn, mateuszlitwin

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[k82cn]

/ok-to-test