Add flatten-maven-plugin for Sonatype Central Portal (#233)

Signed-off-by: Adrian Cole <adrian@tetrate.io>

Author: codefromthecrypt

.gitignore

@@ -24,3 +24,6 @@ target/
 .classpath
 .project
 .settings/
+
+# flatten-maven-plugin
+.flattened-pom.xml

pom.xml

@@ -100,6 +100,7 @@
     <maven-surefire-plugin.argLine />
     <maven-failsafe-plugin.argLine />
 
+    <flatten-maven-plugin.version>1.7.3</flatten-maven-plugin.version>
     <go-offline-maven-plugin.version>1.2.8</go-offline-maven-plugin.version>
     <!-- TODO: cleanup any redundant ignores now also in the 4.0 release (once final) -->
     <license-maven-plugin.version>4.6</license-maven-plugin.version>
@@ -300,6 +301,12 @@
           <artifactId>central-publishing-maven-plugin</artifactId>
           <version>${central-publishing-maven-plugin.version}</version>
         </plugin>
+
+        <plugin>
+          <groupId>org.codehaus.mojo</groupId>
+          <artifactId>flatten-maven-plugin</artifactId>
+          <version>${flatten-maven-plugin.version}</version>
+        </plugin>
       </plugins>
     </pluginManagement>
 
@@ -531,6 +538,30 @@
       <id>release</id>
       <build>
         <plugins>
+          <plugin>
+            <groupId>org.codehaus.mojo</groupId>
+            <artifactId>flatten-maven-plugin</artifactId>
+            <configuration>
+              <flattenMode>ossrh</flattenMode>
+            </configuration>
+            <executions>
+              <execution>
+                <id>flatten</id>
+                <phase>process-resources</phase>
+                <goals>
+                  <goal>flatten</goal>
+                </goals>
+              </execution>
+              <execution>
+                <id>flatten.clean</id>
+                <phase>clean</phase>
+                <goals>
+                  <goal>clean</goal>
+                </goals>
+              </execution>
+            </executions>
+          </plugin>
+
           <plugin>
             <groupId>org.sonatype.central</groupId>
             <artifactId>central-publishing-maven-plugin</artifactId>