influxdb sevice: make postStart test work with non-localhost configurations

[womfoo]

Motivation for this change

The influxdb postStart script fails when its http port is bound to non-127.0.0.1 addresses ie.

  services.influxdb = {
    enable = true;
    extraConfig = {
      http = {
	bind-address = "192.168.1.10:8086";
      };
    };
  };

We currently default to :8086 and this change only uses 127.0.0.1 when the ip is missing from the user's configuraton.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[bjornfor]

CC maintainers (of influxdb package): @offlinehacker @zimbatm

[zimbatm]

@womfoo sounds good, I refactored the change a bit

[womfoo]

@zimbatm looks cleaner now. Thanks!