pgmanage: the data_root option is renamed to sql_root

[basvandijk]

Motivation for this change

Without this we get the following error message:

Apr 01 17:22:35 bassbox pgmanage[1009]: Sun, 01 Apr 2018 17:22:35 PID: 1009
  FILE: common_config.c:126:handler() ERROR === Unknown config section/name:  data_root

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
  • other Linux distributions
• 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.

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