Add appassembler-maven-plugin to *DB Plugin Mgmt

Fixes following errors seen during build:

e.g.

[WARNING] 'build.plugins.plugin.version' for org.codehaus.mojo:
  appassembler-maven-plugin is missing. @ org.apache.asterix:asterix-server
  :[unknown-version], .../asterixdb/asterixdb/asterix-server/pom.xml, line
  224, column 15

Change-Id: I0c79e481e8a74cdf44552536c997f347e4580b14
Reviewed-on: https://asterix-gerrit.ics.uci.edu/1579
Sonar-Qube: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
BAD: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Till Westmann <tillw@apache.org>
diff --git a/asterixdb/pom.xml b/asterixdb/pom.xml
index 196fb18..13c1741 100644
--- a/asterixdb/pom.xml
+++ b/asterixdb/pom.xml
@@ -421,6 +421,11 @@
             <artifactId>maven-scm-plugin</artifactId>
             <version>1.9.5</version>
         </plugin>
+        <plugin>
+          <groupId>org.codehaus.mojo</groupId>
+          <artifactId>appassembler-maven-plugin</artifactId>
+          <version>2.0.0</version>
+        </plugin>
       </plugins>
     </pluginManagement>
   </build>