No zookeeper packages on CentOS slaves

Change 12c4749a tried to install zookeeper on all Ubuntu and CentOS
unit test slaves, but there is not actually a zookeeper-server
package in CentOS or EPEL. Install only on Ubuntu instead.

Change-Id: Ia50f016ef8d53dada63dd3218c7d731016b94383
Correct distro package names for libtidy

* modules/jenkins/manifests/params.pp: Commit 2654943 got the
package names for libtidy backwards between Red Hat and Debian
distro families, so switch them to be correct.

Change-Id: Iaf6290630f679cb64a4b6b17d37d840992988976
Merge "No zookeeper packages on CentOS slaves"
Merge "Correct distro package names for libtidy"