travis: Rename DOCKER_CMD to LINUX_CMD

The new name is arguably more informative, especially now
that we have MACOS_CMD and knowing that MINGW_CMD will
be introduced later on.

We still use DOCKER_CMD when calling Docker, and we assign
the actual script on a per-image basis. Having this
separation will help us when we introduce MinGW builds.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

.travis.yml

@@ -12,11 +12,13 @@ matrix:
       env:
         - IMAGE=ubuntu:18.04
         - DISTCHECK_CONFIGURE_FLAGS="--with-init-script=systemd"
+        - DOCKER_CMD="$LINUX_CMD"
     - services:
         - docker
       env:
         - IMAGE=ubuntu:16.04
         - DISTCHECK_CONFIGURE_FLAGS="--with-init-script=upstart"
+        - DOCKER_CMD="$LINUX_CMD"
     - compiler: clang
       language: c
       os: osx
@@ -42,7 +44,7 @@ git:
 env:
   global:
     - VIR_TEST_DEBUG=1
-    - DOCKER_CMD="
+    - LINUX_CMD="
         apt-get update &&
         apt-get install -y \$PACKAGES &&
         ./autogen.sh &&