Fix models images

[Hedingber]

When building the models and models legacy images this occured when trying to install horovod:

    -- Could NOT find Mxnet (missing: Mxnet_LIBRARIES) (Required is at least version "1.4.0")
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    ModuleNotFoundError: No module named 'mxnet'
    CMake Error at cmake/Modules/FindMxnet.cmake:59 (string):
      string no output variable specified
    Call Stack (most recent call first):
      horovod/mxnet/CMakeLists.txt:12 (find_package)

Full log:
models-image-bug.txt

So added installation of mxnet in the previous step