Adds pause server extension for V3 API

Moves the pause/unpause server functionality out of admin_actions into
its own extension. This part of the blueprint v3-api-admin-actions-split
allows more selective enablement of features contained in the admin
actions extension.

Note that XML api samples are no longer generated because
bp remove-v3-xml-api has been approved.

Partially implements bp v3-api-admin-actions-split
DocImpact: Adds os-pause-server extension and moves pause/unpause
functionality out of os-admin-actions into this new extension

Change-Id: Ib9cce57e2ff1270a82b9d7e39b23ec6b532b9e77
Merge "Adds pause server extension for V3 API"