Build libvirt-exporter for trusty

Change-Id: I2a1138625cd64152b6767e89f3028213d2f7e091
diff --git a/build-debian-packages-libvirt-exporter.groovy b/build-debian-packages-libvirt-exporter.groovy
index ab83db6..d373961 100644
--- a/build-debian-packages-libvirt-exporter.groovy
+++ b/build-debian-packages-libvirt-exporter.groovy
@@ -31,7 +31,7 @@
             }
         }
 
-        def img = dockerLib.getImage("tcpcloud/debian-build-ubuntu-xenial")
+        def img = dockerLib.getImage("tcpcloud/debian-build-ubuntu-${DIST}")
         stage("build package") {
             img.inside("-u root:root") {
                 sh("apt-get update && apt-get install ruby ruby-dev && gem install fpm")