Do requirements for `bootstrap_expect` more softer: ignore the option in the client mode

#### Feature Description
On this page: https://developer.hashicorp.com/consul/docs/agent/config/cli-flags#_bootstrap_expect

We see that `server = true` is required. To my mind you can ignore this option and issue the warning `bootstrap_expect is ignored, because again was run in client mode`

#### Use Case(s)

This allows to reuse configuration files for agent in client or server mode with minimal changes.