Use plain routes list for '/os-keypairs' endpoint instead of stevedore

This patch adds '/os-keypairs' related routes by a plain list, instead
of using stevedore. After all the Nova API endpoints moves to
the plain routes list, the usage of stevedore for API loading will be
removed from Nova.

Also note that the original 'ProjectMapper' use the 'routes.Mapper.resource'
to create a set of routes for a resource which comform to the Atom
publishing protocol. It includes some of URL mappings we didn't document
before. This patch will remove those URL mappings. For the detail,
please reference:
http://lists.openstack.org/pipermail/openstack-dev/2017-March/114736.html

Partial-implement-blueprint api-no-more-extensions-pike
Change-Id: I9b18e7acce37ba935f4302cc89c3aeccc4f50915

setup.cfg

@@ -98,7 +98,6 @@ nova.api.v21.extensions =
     instance_actions = nova.api.openstack.compute.instance_actions:InstanceActions
     instance_usage_audit_log = nova.api.openstack.compute.instance_usage_audit_log:InstanceUsageAuditLog
     ips = nova.api.openstack.compute.ips:IPs
-    keypairs = nova.api.openstack.compute.keypairs:Keypairs
     limits = nova.api.openstack.compute.limits:Limits
     migrations = nova.api.openstack.compute.migrations:Migrations
     multiple_create = nova.api.openstack.compute.multiple_create:MultipleCreate