Add centos7 based optional midonet containers

This patch adds building centos7 images for the midonet specific
containers and makes it possible to run them alongside the
ubuntu based cassandra and zookeeper ones.

To try it out:

echo >> fig.yml <<EOF
zookeeper:
  image: midonet/zookeeper
cassandra:
  image: midonet/cassandra
midolman:
  image: midonet/midolman:2014.11-rc3_centos
  links:
  - "zookeeper:zk"
  - "cassandra:cs"
  privileged: true
api:
  image: midonet/midonet-api:2014.11-rc3_centos
  links:
  - "zookeeper:zk"
  ports:
  - "8080:8080"
EOF

$ docker run -ti --link midonetdockerfile_api_1:api midonet/midonet-cli:2014.11-rc3_centos midonet-cli