Fixes #5012: Postgresql role already exists errors.

Problem appears to have been related to use of puppet ~> notification arrow,
which seems to cause the postgresql module to attempt to re-create the roles
again. In this case we do not appear to want or need notifications sent if a
chained task makes any changes (which it almost certainly will running this
installer the first time). Switching to the simpler -> chaining arrow corrects
the problem and maintains dependency ordering.

WARNING: I am not entirely sure what I'm doing here. However looking at the puppet docs I don't think the ~> was warranted and likely the result of some copy paste. There are probably a lot of other areas where it's being used unnecessarily. 

http://docs.puppetlabs.com/puppet/latest/reference/lang_relationships.html#chaining-arrows

Let me know what you think.