Fix Keypair exception messages

`InvalidKeyPair` exceptions weren't raising the
proper error message. The reason was that the
error message was passed in as an `explanation`
keyword which isn't understood by `NovaException`,
and so is thrown away.

This is part of a patch set to improve keypair
error message reporting in both the Openstack and
EC2 APIs.

References bug 1187952
Change-Id: Ide319cb126955927d623972dffd573cd4c4314df
Merge "Fix Keypair exception messages"