(FACT-234) Partitions Fact

This fact adds a structured fact that reveals details of the partitions on the machine including things like UUID, size, mount point and filesystem.

This PR supersedes PR #560
This can be converted into a `confine` block:

```ruby
Facter.add(:partitions) do
  confine do
    Facter::Util::Partitions.available?
  end
  # ...
end
```