(maint) Update ruby sitelibdir path for PE 3
(maint) Update ruby sitelibdir path for PE 3

Previously pe-puppetdb-terminus was installed to the ruby 1.8 sitelibdir for
pe-ruby1.8. In PE 3, the ruby sitelibdir is the same for all dists. This commit
sets it as a constant and uses that path to install the terminus files.

puppetlabs/puppetdb,wkalt/puppetdb,kbrezina/puppetdb,rbrw/puppetdb,rbrw/puppetdb,jantman/puppetdb,shrug/puppetdb,ajroetker/puppetdb,senior/puppetdb,wkalt/puppetdb,jantman/puppetdb,mullr/puppetdb,shrug/puppetdb,waynr/puppetdb,shrug/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,shrug/puppetdb,kbrezina/puppetdb,ajroetker/puppetdb,kbarber/puppetdb,melissa/puppetdb,cprice404/puppetdb,rbrw/puppetdb,ajroetker/puppetdb,johnduarte/puppetdb,highb/puppetdb,grimradical/puppetdb,waynr/puppetdb,kbarber/puppetdb,melissa/puppetdb,mullr/puppetdb,senior/puppetdb,puppetlabs/puppetdb,highb/puppetdb,johnduarte/puppetdb,johnduarte/puppetdb,cprice404/puppetdb,cprice404/puppetdb,rbrw/puppetdb,wkalt/puppetdb,kbrezina/puppetdb,waynr/puppetdb,puppetlabs/puppetdb,mullr/puppetdb,mullr/puppetdb,kbarber/puppetdb,highb/puppetdb,ajroetker/puppetdb,rbrw/puppetdb,senior/puppetdb,johnduarte/puppetdb,mullr/puppetdb,melissa/puppetdb,kbarber/puppetdb,highb/puppetdb,jantman/puppetdb,senior/puppetdb,waynr/puppetdb,wkalt/puppetdb,grimradical/puppetdb,grimradical/puppetdb,grimradical/puppetdb,kbrezina/puppetdb