Replaces deprecated :keypair for :key_name.

When using a ssh keypair with `knife server create` it will output the
following warning:

> [fog][DEPRECATION] :keypair has been depreciated. Please use :key_name
> instead.

This commit fixes that warning.
Merge pull request #86 from squaresurf/keypair-deprecated

Replaces deprecated :keypair for :key_name.