Allow EC2 API endpoint to be disabled

I would like to provide ability to disable the EC2 API endpoint service
in cookbooks, the ec2 service and endpoint will not be created if the
attribute node['openstack']['compute']['ec2_enabled'] is 'false', the
ec2 section in api-paste.ini will not be generated as well.

Change-Id: I373f9b6821995b48ef29ae16f8273942c18913d7