Make use of ConfigEntry Meta when registering gateways

[shoenig]

hashicorp/consul#8755 adds on Consul's new ability to append metadata on ConfigEntry definitions. We should make use of meta when managing ingress-gateway config entries (and soon the other gateway types) to have Nomad clusters coordinate who is the "owner" of the config entry, rather than having every region re-write the config entry.

[rboyer]

Actually for specific structs it was already done in hashicorp/consul#8596. hashicorp/consul#8755 is about adding an interface method for reading the metadata generically.