OHAI-541: return a response even when ec2 metadata is 404

During EC2 metadata extraction, a given node in the metadata tree
may result in a '404 Not Found' message, causing the entire EC2 plugin
to fail, and returning an empty object.

Since it's better to have most of the metadata returned, this allows
the 404 to continue, and others should continue to fail.
OHAI-541: update fetch_metadata and metadata_get

https://tickets.opscode.com/browse/OHAI-541

Updates per review. Response to 404 is now nil, and tests have been
updated to reflect that. Took a stab at starting some docs on the
metadata_get method, and removed a mis-paste in the license from 3 years
ago.
OHAI-541: catch more opportunities for invalid returns

https://tickets.opscode.com/browse/OHAI-541

Post-review, a few more locations that have some potential to return
an invalid value when called, now guarded with some valid response.
Merge pull request #332 from miketheman/OHAI-541

OHAI-541: return a response even when ec2 metadata is 404