Cinder has supported to query project id of volume
and snapshot, for consistency, this will introduce
backup project attribute in querying operation.
APIImpact
Add 'os-backup-project-attr:project_id: xxxx' in querying
response.
Implements: blueprint: backup-tenant-attribute-support
Change-Id: I6fde17baffe88ab4d4e69dcc2fefdbcb8d7a4dc5
