Updates to make Zookeeper work (#3424)

Netcat was not included in the centos image.
Also the zookeeper scripts were not executable.

Author: nicknezis

docker/dist/Dockerfile.dist.centos7

@@ -22,6 +22,7 @@ RUN yum -y install python; yum clean all
 RUN yum -y install unzip; yum clean all
 RUN yum -y install which; yum clean all
 RUN yum -y install curl; yum clean all
+RUN yum -y install nmap-ncat; yum clean all
 RUN yum -y install python-setuptools; yum clean all
 RUN easy_install supervisor
 RUN echo_supervisord_conf > /etc/supervisord.conf