[commands/check] Add --discovery-instances option to agent check command

[AlexandreYang]

What does this PR do?

Add discovery-instances option to agent check command

Currently, the agent check command waits for at least 1 check instance.
If discovery-instances is used, the agent check will wait for at least the specified number of check instances.

Motivation

Needed for e2e testing listeners (snmp listener) that schedules multiple check instances.

Additional Notes

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
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• Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
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• Changed code has automated tests for its functionality.
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• If applicable, the config template has been updated.