Add spec test showing is_pe detection failure

Add a new spec test providing some initial coverage of
puppetdb::server demontrating the failure to correctly
detect a PE environment.
Fix defined() call to correctly test is_pe

defined('$::is_pe') will never evaluate to true, even if
the is_pe fact is defined. defined('$is_pe') will return the
correct results.
Merge pull request #146 from seanmil/fix_pe_detect

Fix detection of a PE-based PuppetDB