node: fix EasyEC2 instantiation in __init__

Pass in the access key and secret access key to EasyEC2 rather than
using None for both args and swapping out the underlying connection from
the host instance object. This ultimately makes the credentials
available via the EasyEC2 instance where the other approach was leaving
these values None and relying on the underlying connection already
having these values. This fix is mostly useful for folks writing plugins
that need to fetch the current AWS credentials from an EasyEC2 object
(e.g. boto config plugin).

closes gh-302