gitlab-ci: Switch to Ubuntu 18.04 image

In order to run all filesystem tests we need to have newer ext4 tools,
move up to Ubuntu 18.04 'bionic' for our base.  We need to change
slightly how we invoke the provided grub-mkimage.  This will also make
future python3 work easier.

Signed-off-by: Tom Rini <trini@konsulko.com>