Refactor EC2 keypairs exception

According to the following document:

http://docs.amazonwebservices.com/AWSEC2/latest/APIReference/api-error-codes.html

The EC2 API sends specific error codes when keypair listing or
keypair generation fails, specifically the error codes are

InvalidKeyPair.Duplicate - duplicate keypair
InvalidKeyPair.Format - keypair format invalid
InvalidKeyPair.NotFound - keypair not found
ResourceLimitExceeded - keypair quota exceeded

Refactored create_key_pair and describe_key_pair so that
it throws the correct EC2 error codes according to the
public EC2 API specs.

Also removed some duplicate exceptions as well.

Fixes LP: 1072318

DocImpact

Change-Id: Ib7045e49211d9300e9cb6ca0bfe80e569d635c9b
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "Refactor EC2 keypairs exception"