Warn user if bootstrap_exepect is set to an even number

[lgfa29]

Proposal

bootstrap_expect determines how many servers are required to initiate a cluster. It's recommended that this value be an odd number (usually 3 or 5) to avoid split-bran scenarios that could lead to loss of leadership.

Since we know there are potential problems with setting this value as an even number, Nomad should warn users to avoid using these values. This is particularly troublesome for new users, as they may not be aware of this.

In a future release this could even prevent the agent from even starting, but starting with a warning helps maintain backwards compatibility.

Use-cases

Help prevent users (specially new users) from starting Nomad with potentially risky configuration values.

Attempted Solutions

N/A

[github-actions]

I'm going to lock this issue because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.