Keep existing FACTERLIB value

According to http://docs.puppetlabs.com/guides/custom_facts.html
FACTERLIB variable behaves like every other load path. This change keeps
track of the already existing $FACTERLIB (if there's any) by
concatenating packstack's one to it.

Change-Id: I10ef59d5b8ee17978782a7e2a56ce2af7da72ae9