PuppetDB installing postgresql is not optional

The puppetlabs-puppetdb module installs PostgreSQL, and this is not optional.

This means it is not possible to install postgresql separately with a tuned configuration for your environment, or in a manner that could be reused by other services.

Either make the installation of the PuppetDB optiona via a parameter, or remove it completely.

By parameter:
```
class

{'postgresql::server': }

class

{'puppetdb': install_postgresql => false, }

```

..or not at all:
```
class

{'postgresql::server': }

class

{'puppetdb': }

```