Allow UI to query client directly

Nomad web UI currently fails when querying client nodes for allocation
state end endpoints, due to CORS policy.

The issue is that CORS requests that are marked `withCredentials` need
the http server to include a `Access-Control-Allow-Credentials` [1].

But Nomad Task Logs and filesystem requests include authenticating
information and thus marked with `credentials=true`[2][3].

It's worth noting that the browser currently sends credentials and
authentication token to servers anyway; it's just that the response is
not made available to caller nomad ui javascript.  For task logs
specifically, nomad ui retries again by querying the web ui address
(typically pointing to a nomad server) which will forward the request
to the nomad client agent appropriately.

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials
[2] 101d0373ee/ui/app/components/task-log.js (L50)
[3] 101d0373ee/ui/app/services/token.js (L25-L39)

command/agent/http.go

@@ -41,9 +41,10 @@ var (
 
 	// allowCORS sets permissive CORS headers for a handler
 	allowCORS = cors.New(cors.Options{
-		AllowedOrigins: []string{"*"},
-		AllowedMethods: []string{"HEAD", "GET"},
-		AllowedHeaders: []string{"*"},
+		AllowedOrigins:   []string{"*"},
+		AllowedMethods:   []string{"HEAD", "GET"},
+		AllowedHeaders:   []string{"*"},
+		AllowCredentials: true,
 	})
 )