Add Nomad RetryConfig to agent template config

[DerekStrickland]

Closes #13019

This PR introduces a retry configuration option for Nomad dependencies in Consul Template. For example, Nomad native service discovery and secure variables will make calls to the Nomad API. If connectivity is lost between the client and the servers, these calls will fail and the task may fail or run with stale data forever depending on the type of error. Operators can now specify a nomad_retry stanza in the same way they can for Vault or Consul. The default config attempts is set to 0 which allows for infinite retries.

[lgfa29]

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