fix a problem with variable scoping.

It's possible that this was introduced in chef 12, but I'm getting an
errors when using the x509 resource saying:
NoMethodError: undefined method `cert' for Chef::Resource::File
and
NoMethodError: undefined method `key' for Chef::Resource::File
The problem is that inside the file resource, self refers to the file
resource itself, so we need locally scoped variables to hold the data it
needs.