Introduce SecurityGroupLimitExceeded EC2 error code

SecurityGroupLimitExceeded is a valid EC2 error code. Add it as an ec2_code
attribute to the exception of same name and use it where appropriate
instead of useless EC2APIError.

Implements: blueprint ec2-error-codes

Change-Id: I1558b9f69c912dab9d829aacd5c5aae1c37f12b9
Merge "Introduce SecurityGroupLimitExceeded EC2 error code"