Since people have been asking how to blend the nodes together like in the example above here is one way to do it.
Make the white part of the color ramp darker grey depending on the scale of the bubbles (voronoi) to adjust their height then blend them with a color mix set to lighten.
The second color ramp from the original example controls how many (or how dense) you want the bubbles to be.
To get a flat area your best bet is to add a white image texture and mix it in with a color mix set to darken, then go into texture paint and use a black brush to paint out the bubbles you don't want. Obviously the more effort you put into this step the better it will look. I was sloppy here but you can see it works pretty well.