Define version of javadoc plugin used to generate site

This silences the Maven warning "'reporting.plugins.plugin.version' for
org.apache.maven.plugins:maven-javadoc-plugin is missing"

Change-Id: I036210fefb6bd81ca04fcd91aaba0f9d0c1e8862
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

pom.xml

@@ -189,6 +189,7 @@
     <jetty-version>7.6.14.v20131031</jetty-version>
     <clirr-version>2.6.1</clirr-version>
     <httpclient-version>4.1.3</httpclient-version>
+    <maven-javadoc-plugin-version>2.9.1</maven-javadoc-plugin-version>
 
     <!-- Properties to enable jacoco code coverage analysis -->
     <sonar.core.codeCoveragePlugin>jacoco</sonar.core.codeCoveragePlugin>
@@ -270,7 +271,7 @@
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-javadoc-plugin</artifactId>
-          <version>2.9.1</version>
+          <version>${maven-javadoc-plugin-version}</version>
         </plugin>
 
         <plugin>
@@ -480,6 +481,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
+        <version>${maven-javadoc-plugin-version}</version>
         <configuration>
         </configuration>
       </plugin>