travis: Add CentOS 7 build

Now that we use pre-built Docker images, it's very easy
to extend our test matrix; adding CentOS 7 is a good start.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -13,6 +13,12 @@ matrix:
         - IMAGE="ubuntu-18"
         - DISTCHECK_CONFIGURE_FLAGS="--with-init-script=systemd"
         - DOCKER_CMD="$LINUX_CMD"
+    - services:
+        - docker
+      env:
+        - IMAGE="centos-7"
+        - DISTCHECK_CONFIGURE_FLAGS="--with-init-script=upstart"
+        - DOCKER_CMD="$LINUX_CMD"
     - compiler: clang
       language: c
       os: osx