pypi package msgpack-python is deprecated, drop explicit install

according to https://pypi.org/project/msgpack-python/ it's
deprecated and should use msgpack.

and this is supported by looking at a build source container
image and noting that pip lists

msgpack                 0.6.0
msgpack-python          0.5.6

also msgpack is pulled in via dependency of oslo.serialization

We should be able to rely on it coming in via that dependency
automaticly

Change-Id: Id2706b3b38e57daf1166f98a59dc3ecd30fd4fa7

docker/openstack-base/Dockerfile.j2

@@ -34,7 +34,6 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
             'openssl',
             'python2-pyOpenSSL',
             'pyparsing',
-            'python2-msgpack',
             'python2-cryptography',
             'python2-debtcollector',
             'python2-eventlet',
@@ -313,7 +312,6 @@ RUN curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py \
         'keystonemiddleware',
         'kombu',
         'monotonic',
-        'msgpack-python',
         'mysqlclient',
         'netaddr',
         'netifaces',