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.