added "which" package to rhel/centos provisioning

Author: portertech

lib/kitchen/driver/docker.rb

@@ -112,7 +112,7 @@ def dockerfile
         when 'rhel', 'centos'
           <<-eos
             RUN yum clean all
-            RUN yum install -y sudo openssh-server openssh-clients curl
+            RUN yum install -y sudo openssh-server openssh-clients which curl
             RUN ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key
             RUN ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key
           eos