docs/running-on-mesos.md: state default values in default column

This PR simply uses the default value column for defaults.

Author: Felix Bechstein <felix.bechstein@otto.de>

Closes #8810 from felixb/fix_mesos_doc.

docs/running-on-mesos.md

@@ -332,21 +332,21 @@ See the [configuration page](configuration.html) for information on Spark config
 </tr>
 <tr>
   <td><code>spark.mesos.principal</code></td>
-  <td>Framework principal to authenticate to Mesos</td>
+  <td>(none)</td>
   <td>
     Set the principal with which Spark framework will use to authenticate with Mesos.
   </td>
 </tr>
 <tr>
   <td><code>spark.mesos.secret</code></td>
-  <td>Framework secret to authenticate to Mesos</td>
+  <td>(none)/td>
   <td>
     Set the secret with which Spark framework will use to authenticate with Mesos.
   </td>
 </tr>
 <tr>
   <td><code>spark.mesos.role</code></td>
-  <td>Role for the Spark framework</td>
+  <td>*</td>
   <td>
     Set the role of this Spark framework for Mesos. Roles are used in Mesos for reservations
     and resource weight sharing.
@@ -354,7 +354,7 @@ See the [configuration page](configuration.html) for information on Spark config
 </tr>
 <tr>
   <td><code>spark.mesos.constraints</code></td>
-  <td>Attribute based constraints to be matched against when accepting resource offers.</td>
+  <td>(none)</td>
   <td>
     Attribute based constraints on mesos resource offers. By default, all resource offers will be accepted. Refer to <a href="http://mesos.apache.org/documentation/attributes-resources/">Mesos Attributes & Resources</a> for more information on attributes.
     <ul>