kola/options/do: increase default droplet size
kola/options/do: increase default droplet size

DigitalOcean recently upped the ram in all the droplets for free. Use
beefier instances becuase it costs the same. This should also fix the
coreos.toolbox.dnf-install test that previously failed due to being
OOM-killed

dm0-/mantle,dm0-/mantle,glevand/coreos--mantle,glevand/coreos--mantle