x/build: tracking bug for Kubernetes buildlet pool support

[bradfitz]

Tracking bug for adding a Kubernetes buildlet pool to the build coordinator.

Kubernetes would only work for the linux and nacl builders, and only for amd64 and 386, but that's a lot of our builders, and it'd mean they'd start up much faster.

/cc @proppy @adg

[bradfitz]

First bits:
https://go-review.googlesource.com/9981
https://go-review.googlesource.com/14384
https://go-review.googlesource.com/14390

[gopherbot]

CL https://golang.org/cl/14396 mentions this issue.

[gopherbot]

CL https://golang.org/cl/14532 mentions this issue.

[gopherbot]

CL https://golang.org/cl/15041 mentions this issue.

[gopherbot]

CL https://golang.org/cl/15258 mentions this issue.

[gopherbot]

CL https://golang.org/cl/15285 mentions this issue.

[gopherbot]

CL https://golang.org/cl/16101 mentions this issue.