Global: Rename default_linux_image to default_linux_container

Now that we have introduced default_linux_vm, the naming becomes
somewhat confusing, since there are two different images specified
for two different reasons.

By naming one of them _vm and the other _container
we surface their intended usage in their variable name.

Continuous-integration-options: PatchCheck.ignore-multi-package

Signed-off-by: Mike Beaton <mjsbeaton@gmail.com>