cmd/coordinator, dashboard: switch to new VMWare-based OS X builders

Ignore the old darwin-{amd64,386}-10_10 builders. Don't give them an
error, but pretend they don't exist.

Also: switch trybots from OS X 10.10 to OS X 10.11, and re-enable
sharding. Let's hope for the best. See golang/go#12979.

This also enables subrepo tests for all OS X versions.

darwin-386-* is currently offline, pending some golang/go#17009

Updates golang/go#9495 (OS X virtualization)

Change-Id: I4d53a79087404b5e8051d1aff0c668a92625f442
Reviewed-on: https://go-review.googlesource.com/28583
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed