DNS options for network namespaced allocs

With task groups now able to share a network namespace, users may need to override the DNS options (resolver, hosts, etc) that are inherited from the host network namespace. Nomad should provide a way for users to do this.

No files, tests, or entry points are identified in the issue. Start by locating how network-namespaced task groups inherit DNS settings from the host namespace, then define the supported user-facing override behavior and tests that demonstrate resolver and hosts configuration can be changed.

Written by the indexing model from the issue text.