improve validation conditions for MetalLB BGP Peers

[kakkotetsu]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

When I replace MetalLB speaker by calico Service LoadBalancer IP advertisement, metallb_peers is NOT mandatory. (docs / MetalLB / BGP Mode)
And when metallb_peers is mandatory(metallb_protocol is bgp && metallb_speaker_enabled is true), it is also better to make sure the metallb_peers is not empty({}).

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[MetalLB] Improve validation conditions for BGP Peers

[k8s-ci-robot]

Hi @kakkotetsu. Thanks for your PR.

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[oomichi]

looks good to improve the check.

/cc @cristicalin
/ok-to-test
/lgtm

[cristicalin]

Thanks for taking care of this @kakkotetsu

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cristicalin, kakkotetsu

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