[SPARK-8083] [MESOS] Use the correct base path in mesos driver page.

Author: Timothy Chen <tnachen@gmail.com>

Closes #6615 from tnachen/mesos_driver_path and squashes the following commits:

4f47b7c [Timothy Chen] Use the correct base path in mesos driver page.

(cherry picked from commit bfbf12b3)
Signed-off-by: Andrew Or <andrew@databricks.com>

core/src/main/scala/org/apache/spark/deploy/mesos/ui/DriverPage.scala

@@ -68,7 +68,7 @@ private[ui] class DriverPage(parent: MesosClusterUI) extends WebUIPage("driver")
         retryHeaders, retryRow, Iterable.apply(driverState.description.retryState))
     val content =
       <p>Driver state information for driver id {driverId}</p>
-        <a href="/">Back to Drivers</a>
+        <a href={UIUtils.prependBaseUri("/")}>Back to Drivers</a>
         <div class="row-fluid">
           <div class="span12">
             <h4>Driver state: {driverState.state}</h4>