JENKINS-22960. wait for build to be finalized

timing issues can occur if a build is complete but not finalized.
e.g. trying to access artifacts before they are stored on disk may be problematic.
```
Results :

Tests run: 41, Failures: 0, Errors: 0, Skipped: 0

[INFO] 
[INFO] --- maven-license-plugin:1.4:process (default) @ build-flow-plugin ---
[INFO] Generated /Users/damien/Work/github/build-flow-plugin/target/build-flow-plugin/WEB-INF/licenses.xml
[INFO] 
[INFO] --- maven-hpi-plugin:1.74:hpi (default-hpi) @ build-flow-plugin ---
[INFO] Exploding webapp...
[INFO] Copy webapp webResources to /Users/damien/Work/github/build-flow-plugin/target/build-flow-plugin
[INFO] Assembling webapp build-flow-plugin in /Users/damien/Work/github/build-flow-plugin/target/build-flow-plugin
[INFO] Generating hpi /Users/damien/Work/github/build-flow-plugin/target/build-flow-plugin.hpi
[INFO] Generating /Users/damien/Work/github/build-flow-plugin/target/build-flow-plugin/META-INF/MANIFEST.MF
[INFO] Building jar: /Users/damien/Work/github/build-flow-plugin/target/build-flow-plugin.hpi
[INFO] Building jar: /Users/damien/Work/github/build-flow-plugin/target/build-flow-plugin.jar
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 06:40 min
[INFO] Finished at: 2014-05-11T22:27:56-08:00
[INFO] Final Memory: 33M/81M
[INFO] ------------------------------------------------------------------------
```