Ensure server stop in case of start timeout

virtualnav-webdriver-tests.start-unix:
(...)
[INFO]      [exec] Server started with process ID 18575.
[INFO]      [exec] ..........................................................................................................................................................................................................................................................................................................
[INFO]      [exec] Starting process is taking too long - giving up.
[INFO]      [exec] Some commands failed:
[ERROR] 
BUILD FAILED - at 23/10/14 15:31
/home/julien/ws-ssd/nuxeo/marketplace/marketplace-virtualnavigation/ftest/webdriver/target/nuxeo-ftest.xml:1471: exec returned: 1

Total time: 5 minutes 11 seconds
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] Virtual Navigation packaging parent POM ............ SUCCESS [  1.300 s]
[INFO] Virtual Navigation Marketplace Package ............. SUCCESS [ 14.174 s]
[INFO] Virtual Navigation Functional Tests with WebDriver . FAILURE [05:13 min]
[INFO] Virtual Navigation Functional Tests with Selenium .. SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE