Adds create backup server extension for the V3 API

Moves create_backup functionality out of admin_actions
into its own extension. This is part of blueprint v3-admin-actions-split
which 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-create-backup extension and moves the
create_backup functionality out of os-admin-actions into
this new extension.

Change-Id: I4858a06df20c552bd55ff2841adbcfc761304f42
Merge "Adds create backup server extension for the V3 API"