mariadb-server requires the hostname userspace command

The hostname command is required to allow mariadb to access
the network properly.  Without it, in some cases mariadb cannot
execute name service resolution.

Change-Id: Ia215585faad23012143b63c4d82acd6ba69f59b7

docker/mariadb/Dockerfile

 FROM %%KOLLA_NAMESPACE%%/%%KOLLA_PREFIX%%base
 MAINTAINER Kolla Project (https://launchpad.net/kolla)
 
-RUN yum -y install mariadb-galera-server; yum clean all
+RUN yum -y install mariadb-galera-server hostname; yum clean all
 ADD /entrypoint.sh /entrypoint.sh
 
 VOLUME /var/lib/mysql