Disabling server_rejoin_age_max or its handling

#### Feature Description

When running a single-node consul cluster, there are no other cluster leaders. If the server running consul is down for some time and starts, `server_rejoin_age_max` cause the consul server to not bootstrap.

The meaning of the value is, that the server has been offline for so long, other cluster leaders should have a far better "state" then this one - this is a fine feature for multi node clusters / multi-leader clusters. But for a single node cluster, there is no "better state" - the one starting has the only state available.

It would be great to
- either disable the handling using variable
- automatically disable the handling in a single node cluster - this might be harder to detect (what if it is only a split) - so maybe there should be an opt in var "single_node_only" that would then be usable for other aspects / simplifications too.

#### Use Case(s)

Single node cluster

Thanks!