Improvements have gone into Puppet 5 that obsolete processing done in the puppetdb terminus, meaning we're doing a bunch of extra work there.
With a hard dep on puppet agent 5 we can rely on the new behavior, remove some of the processing in the terminus, with likely performance and maintainability benefits.
For example, the `stringify_` methods in the puppetdb catalog terminus are likely no longer necessary.
- Update the dependency that puppetdb-termini expresses on puppet-agent to be versioned, ie puppet-agent >= 5
Currently, FOSS PDB specifies a dependency on puppet (<< 5.0.0-1puppetlabs1) which may mean that users can't upgrade to Puppet 5 on their FOSS PDB/MOM unless we make this change - PDB 5.0 can be installed only with Puppet Agent 4.10.x or earlier.