[feature] allow reload of Consul config stanza and Consul client

When using TLS for Consul connectivity, it is obviously preferable to use low TTL certificates with frequent renewal. Currently in order to renew the Consul certificates the Nomad client must be restarted. It would be preferable if the Nomad client could use SIGUP in order to reload the Consul client/config like reloading the Nomad TLS certificates. This would be a much less intrusive operation and involve less risk.

Start by tracing the existing SIGUP reload path for Nomad TLS certificates and the Consul configuration and client initialization. The change is complete when renewed Consul TLS certificates can be loaded through SIGUP without restarting the Nomad client, with coverage for the reload behavior.

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