vagrant: add bash completions

[megheaiulian]

Motivation for this change

Adds on-demand bash completions for vagrant

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option 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 nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[samueldr]

 result/bin
 result/bin/vagrant
 result/bin/.vagrant-wrapped
+result/share
+result/share/bash-completion
+result/share/bash-completion/completions
+result/share/bash-completion/completions/vagrant
 result/vagrant-plugins
 result/vagrant-plugins/plugins.json
 result/vagrant-plugins/plugins.d

👍