Introduce InvalidPermission.Duplicate EC2 error code

New SecurityGroupRuleExists exception with 'InvalidPermission.Duplicate'
ec2_code attribute replaced useless EC2APIError.

Note that this error isn't about duplicate group as the code suggests,
but about duplicate permission within a group.

Implements: blueprint ec2-error-codes

Change-Id: I42c94e15a6ca1bc57b10fb683bec17b73de0e0e5
Merge "Introduce InvalidPermission.Duplicate EC2 error code"