Map caps-lock to control on Ubuntu 17.10

You probably want to install the gnome-tweak-tool package

sudo apt install gnome-tweak-tool

and start Tweaks.

Then: Keyboard & Mouse -> Additional Layout Options -> Caps Lock behavior

On Ubuntu 18.04 there's no way to save or exit from the Additional Layout Options, I had to kill the process. gnome-tweak-tool is worthless, but setxkbmap -option ctrl:nocaps worked perfect.