Implement Guest capability for retrieving facts

[Sharpie]

Currently, the only way for PE Build to determine which Guest OS, OS version and architecture it is dealing with is through the detect_installer capability. Unfortunately, this capability bakes all of that info into the name of an installer tarball, which is the only return value:

puppet-enterprise-2015.2.0-el-6-x86_64.tar.gz

In order to allow provisioners to implement more flexible decision making, we need a capability that just gathers facts.