fix panic when Connect mesh gateway doesn't have a proxy block

[lgfa29]

When a gateway -> mesh block is set in a service without a proxy block, Nomad is expected to generate a default proxy configuration. This was being done when the group network mode was bridge, but not when it was host, even though that is a supported option.

Without the proxy configuration, Nomad would panic when trying to generate the Consul API request to register the service.

This PR generates the proxy configuration regardless of the network mode. When in bridge, the previous behaviour is preserved. In host, no special configuration is done, so the default Consul behaviour is used.

Closes #11243

[github-actions]

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