(PE-618) Don't suggest 'postgres' for pe-puppetdb on debian

We're going to be installing pe-postgresql whenever we need to in PE, so we
shouldn't suggest to the user that they also install postgres.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #528 from MosesMendoza/maint/master/dont_suggest_postgres_on_pe

(PE-618) Don't suggest 'postgres' for pe-puppetdb on debian