Add flatten-maven-plugin (fixes: issue aerospike-community#69) (aeros…

…pike-community#71)

* Add flatten-maven-plugin.
Solves the issue of "POM files without a version" caused by the usage of ${revision} property in a multi module project with Maven version below 3.5.
Issue described here:
https://blog.soebes.de/blog/2017/04/02/maven-pom-files-without-a-version-in-it/

Related StackOverFlow questions:
1. https://stackoverflow.com/questions/41086512/maven-issue-to-build-one-module-using-revision-property
2. https://stackoverflow.com/questions/52173260/how-to-resolve-parent-pom-dependency-issue-failed-to-read-artifact-descriptor/52173650#52173650

* Use flattenMode "bom" (Bill of materials) instead of individual pomElements which keeps all optional POM elements that are required for OSS Repository-Hosting and additionally dependencyManagement and properties values.

pom.xml

@@ -199,6 +199,31 @@
                     </execution>
                 </executions>
             </plugin>
+            <plugin>
+                <groupId>org.codehaus.mojo</groupId>
+                <artifactId>flatten-maven-plugin</artifactId>
+                <version>1.2.7</version>
+                <configuration>
+                    <updatePomFile>true</updatePomFile>
+                    <flattenMode>bom</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>
         </plugins>
     </build>
 </project>