all: misc cleanup, docs, actually add Alpine builder

-- move policy of which builders are trybots out of coordinator
   and into dashboard/builders.go.

-- move some GCE-specific code from coordinator.go to gce.go.

-- rename an old "watcher" reference to "gitmirror"

-- add some docs

-- actually add the Alpine builder, missing from https://golang.org/cl/33890

Fixes golang/go#17891

Change-Id: Ia63671ca09aec322ed57b3663e0ac5042cdc56f2
Reviewed-on: https://go-review.googlesource.com/40395
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/dashboard/builders_test.go b/dashboard/builders_test.go
index 96effe5..96f1b17 100644
--- a/dashboard/builders_test.go
+++ b/dashboard/builders_test.go
@@ -20,3 +20,26 @@
 		}
 	}
 }
+
+func TestListTrybots(t *testing.T) {
+	tryBots := TrybotBuilderNames()
+	t.Logf("Builders:")
+	for _, name := range tryBots {
+		t.Logf("  - %s", name)
+	}
+}
+
+func TestHostConfigsAllUsed(t *testing.T) {
+	used := map[string]bool{}
+	for _, conf := range Builders {
+		used[conf.HostType] = true
+	}
+	for hostType := range Hosts {
+		if !used[hostType] {
+			// Currently host-linux-armhf-cross and host-linux-armel-cross aren't
+			// referenced, but the coordinator hard-codes them, so don't make
+			// this an error for now.
+			t.Logf("warning: host type %q is not referenced from any build config", hostType)
+		}
+	}
+}