Changes for gpg ignore Darwin (OSX) support

[edestecd]

The latest changes to class rvm::system ignore Darwin.

GPG is only used by rvm if it it installed on the system. Darwin does not come with gpg, so
exec { 'system-rvm-gpg-key': } will blow up.

I just checked and rvm skips gpg check and proceeds normally on stock OSX.

RVM will skip gpg if not present, so maybe just run this on:
kernel = Linux
make a fact to detect if gpg present
What does everyone prefer?

Furthermore roots home is /var/root on Darwin.

I will start working on some commits...