lib, gapi, cli: Add a wait flag to empty and a new default

Change the default "wait" state for if you run the empty frontend when
there's already an available deploy waiting. You almost certainly want
to start running it right away.

Example:

mgmt etcd

mgmt run --hostname h1 --no-server --tmp-prefix --seeds=http://127.0.0.1:2379 empty
mgmt run --hostname h2 --no-server --tmp-prefix --seeds=http://127.0.0.1:2379 empty

mgmt deploy --no-git --seeds=http://127.0.0.1:2379 lang examples/lang/hello0.mcl

mgmt run --hostname h3 --no-server --tmp-prefix --seeds=http://127.0.0.1:2379 empty

In fact, you don't even need to start up etcd first for this to all
work.

cli/util/args.go

@@ -70,7 +70,9 @@ func LookupSubcommand(obj interface{}, st interface{}) string {
 }
 
 // EmptyArgs is the empty CLI parsing structure and type of the parsed result.
-type EmptyArgs struct{}
+type EmptyArgs struct {
+	Wait bool `arg:"--wait" help:"don't use any existing (stale) deploys"`
+}
 
 // LangArgs is the lang CLI parsing structure and type of the parsed result.
 type LangArgs struct {