Add CORS header to /ping.

This is allows a client to do health checks from JavaScript

Review: https://rb.mesosphe.re/r/573

src/main/scala/mesosphere/chaos/http/PingServlet.scala

@@ -0,0 +1,34 @@
+package mesosphere.chaos.http
+
+import javax.servlet.http.{HttpServletRequest, HttpServletResponse, HttpServlet}
+
+/**
+ * An HTTP servlets which outputs a {@code text/plain} {@code "pong"} response.
+ *
+ * PingServlet from com.codahale.metrics, with CORBS header added.
+ */
+
+class PingServlet extends HttpServlet {
+
+  private final val CONTENT_TYPE = "text/plain"
+  private final val CONTENT = "pong"
+  private final val CACHE_CONTROL = "Cache-Control"
+  private final val NO_CACHE = "must-revalidate,no-cache,no-store"
+  private final val ACCESS_CONTROL_ALLOW_ORIGIN = "Access-Control-Allow-Origin"
+  private final val STAR = "*"
+
+  protected override def doGet(req: HttpServletRequest, resp: HttpServletResponse) {
+    resp.setStatus(HttpServletResponse.SC_OK)
+    resp.setHeader(CACHE_CONTROL, NO_CACHE)
+    resp.setHeader(ACCESS_CONTROL_ALLOW_ORIGIN, STAR)
+    resp.setContentType(CONTENT_TYPE)
+
+    val writer = resp.getWriter
+    try {
+      writer.println(CONTENT)
+    }
+    finally {
+      writer.close()
+    }
+  }
+}

src/main/scala/mesosphere/chaos/http/RestModule.scala

@@ -1,6 +1,6 @@
 package mesosphere.chaos.http
 
-import com.codahale.metrics.servlets.{MetricsServlet, PingServlet}
+import com.codahale.metrics.servlets.MetricsServlet
 import com.google.inject.{Singleton, Provides, Scopes}
 import com.sun.jersey.guice.spi.container.servlet.GuiceContainer
 import com.fasterxml.jackson.jaxrs.json.JacksonJsonProvider