Add support for running on Puppet Enterprise Primary

[bastelfreak]

This enables user to run foreman-proxy on a Puppet Enterprise primary.

[bastelfreak]

Draft because I'm still testing

[evgeni]

Technically, the fact is from the Agent, so if people run OSS Agent on PE Server, this will still break, but I really hope nobody is that crazy? ;-)

[bastelfreak]

So after a very ugly debugging session:

• is_pe exists in stdlib and in puppet enterprise. in PE it's broken and always returns false (lol)
• is_pe from stdlib is true when pe_version is not empty
• pe_version gets the same value as pe_build
• pe_build gets the value from reading /opt/puppetlabs/server/pe_version
• /opt/puppetlabs/server/pe_version is provided by pe-puppet-enterprise-release.rpm
• the puppet-agent.rpm on PE and FOSS is identical
• This patch works on every PE infra node, no matter where the agent is coming from