Allow SONAR_MAVEN_GOAL to be configurable

Allow SONAR_MAVEN_GOAL to be configurable

Per reports on the mailing list Sonar jobs are broken in OpenDaylight.
This is because Sonar release a new plugin which only works with newer
versions of Sonar so until we can upgrade we need to allow flexibility
to configure the Sonar job to override the default sonar:sonar goal.

With this patch it is now possible to override the Sonar goal in JJB
and force an older version of the Sonar plugin to run using a JJB
defaults config or projects variable like this:

  sonar-mvn-goal: org.codehaus.mojo:sonar-maven-plugin:3.3.0.603:sonar

Unfortunately due to the defaults priority of JJB since we are
defining the sonar-mvn-goal parameter inside of the job template it
can only be overrided via a project section.

Reports:
https://lists.opendaylight.org/pipermail/release/2017-December/013119.html
https://lists.opendaylight.org/pipermail/release/2017-December/013223.html

Change-Id: I04512613f39e7c328e0f2f18d9c4729667e86909
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>