Add a rally role

This will be used to benchmark our stack. This role installs rally in a
venv and adds a deployment for it, as well as a benchmark set. Rally is
set to log directly to /var/log/rally.log and the data is kept in a
local sqlite database. For now the rally REST service is not enabled.

To get rally going, add a rally group in hosts and add a host to the
group. Test clusters will get rally by default.