fingerprint: Add retry and failure config to env fingerprinters.

[jrasell]

This change introduces new optional client fingerprinter configuration fields which can be used to control how the env fingerprinters perform retries and whether errors should halt the agent startup.

The retry wrapper is used by the env_aws, env_azure, env_gce, and env_digitalocean fingerprinters and is the handler for retry and error logic on the main fingerprinter. The change is backwards compatible, so running this change without any new config options results in the same behaviour as previously.

• retry_interval: Specifies the time to wait between fingerprint attempts. This will default to 2 seconds.
• retry_attempts: Specifies the maximum number of fingerprint retries to be made. This will default to 0 and can be set to -1 if the operator wants infinite retries.
• exit_on_failure: Determines how the agent handles failure in performing the fingerprint.

The change helps alleviate problems in cloud providers where a machine starts before the metadata service and endpoint is available. In this situation, Nomad timesout the fingerprinter quickly and marks it as skipped, thus assuming we are not running within that environment. Operators can use the new configuration options to handle these race conditions, and wait for the metadata service to be available and respond.

Links

Jira: https://hashicorp.atlassian.net/browse/NMD-1061

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[pkazmierczak]

LGTM! Left some minor typo-related comments, but nothing blocking.