Makes v3 API keypairs extension core

Adds os-keypairs to the core list for the V3 API.

Rename os-keypairs to keypairs as part of this change.

Fixes some incorrect urls in test_keypairs which still included
the tenant id

Updates api samples for servers since keypairs is now core and forms
part of the response by default

+DocImpact

Partially implements blueprint nova-v3-api

Change-Id: I8cdf52be0c8b19f7704848029a78373da8018cab
Merge "Makes v3 API keypairs extension core"