Google-cloud-node: Add Node.js 12 to the CI

[JustinBeckwith]

Looks like Node.js 12 was just released! Let's get it added to the kokoro configs.

[JustinBeckwith]

@bcoe please work with @jkwlui on getting this added :)

[bcoe]

@jkwlui @JustinBeckwith I'd like to roll out the .npmrc updates too; tldr, @jkwlui please sync up with me before we start rolling this out, and we can use the bulk repo update tool.

[nolanmar511]

Is the trampoline image for Node 12 available?

[nolanmar511]

Friendly ping: How is progress on this going?

[JustinBeckwith]

@bcoe I think it's probably a good time to roll this out

[bcoe]

@JustinBeckwith I've been somewhat patiently waiting an update to V8, because there's an odd bug related to test coverage that I'm hoping is fixed up stream ... this is something we can look at prioritizing in the not too distant future.

@jkwlui is the trampoline image available? we just reference the canonical ones through nvm right?

[nolanmar511]

I tried to add tests using a Node 12 trampoline image. As of 7/8, the last time I tried to start running this test, I don't think the image was available (kokoro config is here).

[bcoe]

Node 12 is now available and tested.