Inherit from correct class for WSGI server
Inherit from correct class for WSGI server

Using oslo.service requires the WSGI server class to implement
oslo_service.service.ServiceBase. ec2api.wsgi.Server already implemented
the interface thus defined but lacked the correct parent class.

Change-Id: Iecd2721830821ef75a321fa800d2bbd85b3ce1e1
Closes-Bug: #1562860

stackforge/ec2-api,stackforge/ec2-api,openstack/ec2-api,openstack/ec2-api