dns: simplify newStaticClientConfig implementation

[alexandear]

This PR refactors unexported function newStaticClientConfig in the hostagent/dns package.

The PR silents one modernize (will be enabled later) lint issue:

$ golangci-lint run --enable-only modernize

pkg/hostagent/dns/dns.go:74:3: stringsbuilder: using string += string in a loop is inefficient (modernize)
                s += fmt.Sprintf("nameserver %s\n", ip)
                ^

[jandubois]

@alexandear I'm rerunning the tests right now, but if they continue to fail, then you may need to rebase your branch on the latest master.