Fix checkstyle configuration (iotaledger#990)

checkstyle.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <!DOCTYPE module PUBLIC
-          "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
-          "http://checkstyle.sourceforge.net/dtds/configuration_1_3.dtd">
+          "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+          "https://checkstyle.org/dtds/configuration_1_3.dtd">
 <!--
 
 IOTA checkstyle draft

pom.xml

@@ -20,6 +20,10 @@
         <java-version>1.8</java-version>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <undertow.version>1.4.6.Final</undertow.version>
+        <!-- Setting the checkstyle.config.location here will make checkstyle use this file in every maven target.
+             This variable will be load by checkstyle plugin automatically. It is more global than setting the
+             configLocation attribute on every maven goal. -->
+        <checkstyle.config.location>checkstyle.xml</checkstyle.config.location>
     </properties>
 
     <repositories>
@@ -259,6 +263,8 @@
                     </execution>
                 </executions>
             </plugin>
+            <!-- This checkstyle goal will only be invoked during the validate goal or by running "mvn validate"
+                 manually. It indirectly calls "mvn checkstyle:check" -->
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-checkstyle-plugin</artifactId>
@@ -268,11 +274,10 @@
                         <id>validate</id>
                         <phase>validate</phase>
                         <configuration>
-                            <configLocation>checkstyle.xml</configLocation>
+                            <configLocation>${checkstyle.config.location}</configLocation>
                             <encoding>UTF-8</encoding>
                             <consoleOutput>true</consoleOutput>
                             <failsOnError>true</failsOnError>
-                            <linkXRef>false</linkXRef>
                         </configuration>
                         <goals>
                             <goal>check</goal>
@@ -565,10 +570,12 @@
     </profiles>
     <reporting>
         <plugins>
+            <!-- This checkstyle goal is indirectly called during report generation or by
+                 running "mvn checkstyle:checkstyle" manually-->
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-checkstyle-plugin</artifactId>
-                <version>2.17</version>
+                <version>3.0.0</version>
                 <reportSets>
                     <reportSet>
                         <reports>