Added 'unlimited' config setting for peer_discovery_retry_limit (backport #13673) (backport #13675)

[mergify]

Proposed Changes

The peer discovery code can handle the Retry value to be either a non-negative integer, or the atom 'unlimited'.

-spec discovery_retries(Backend) -> {Retries, RetryDelay} when
      Backend :: backend(),
      Retries :: non_neg_integer() | unlimited,
      RetryDelay :: non_neg_integer().

But the cuttlefish config only allows non neg integers. I suggest we also allow the value 'unlimited' to be set through config.

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This is an automatic backport of pull request #13673 done by [Mergify](https://mergify.com).

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This is an automatic backport of pull request #13675 done by [Mergify](https://mergify.com).