CHEF-5030: improve debian ifconfig provider code

- dynamically create directory resource to create /etc/network/interfaces.d
- better idempotency and accuracy in what we report to the user
- dependency injection in tests that uses real tempfiles instead of mocking File class, etc
Merge pull request #1289 from opscode/lcg/CHEF-5030

CHEF-5030:  improve debian ifconfig provider code