Handle multiple postgresql versions with instances

[SimonHoenscheid]

This issue is a follow up of PR #1450.

@bzed mentioned that it might be beneficial to add the version to the instance name. this would allow the user to have instances of the same name with different versions. A good example is an upgrade to a newer version.

Possible pitfalls:

• package management
• dependency to postgresql::server for defaults
• postgresql::globals expects a single string for $version

might be a good preparation for #1524