sonar: modify sonar-scan.sh script

Prevent execution on non configured env & add sonar target to allowed failures
in travis

Signed-off-by: Nicolas Oliver <dario.n.oliver@intel.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

.travis.yml

@@ -1,3 +1,4 @@
+dist: trusty
 sudo: required
 
 language: cpp
@@ -47,6 +48,8 @@ matrix:
       env: JSONPLAT=ON TARGET=node5
     - compiler: clang
       env: JSONPLAT=ON TARGET=node5
+    - compiler: gcc
+      env: TARGET=sonar-scan
 
 services:
   - docker