Get_keypair() will raise exception if no keypair exists

[anthony198]

Hi, Viglesiasce

In ec2ops.py , if no keypair exists that has the name passed, below code will fail because it try to access it as a list:

def get_keypair(self, name):
    return self.ec2.get_all_key_pairs([name])[0]

[viglesiasce]

Fixed in master. Now throws a more accurate exception when no keypair is found.