Issue description

Currently (18.09/19.03) services.mysql.ensureUsers has no ability to create permissions which allow the "with grant option" to allow full database admin access rights like root:

MariaDB [(none)]> show grants for 'root'@'localhost';
GRANT ALL PRIVILEGES ON . TO 'root'@'localhost' IDENTIFIED VIA unix_socket WITH GRANT OPTION

Steps to reproduce

Create any mysql user using ensureUsers and note there is no way to add "with grant option".

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

• system: "x86_64-linux"
• host os: Linux 4.14.78, NixOS, 18.09.1127.c341f56928c (Jellyfish)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.1.1
• channels(root): "nixos-18.09.945.9779809e050"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos