Volume test for backup:backup_project_attribute

Add RBAC test for "backup:backup_project_attribute" which verifies
that the "os-backup-project-attr:project_id" attribute appears in
the response body only policy enforcement succeeds.

This commit specifically:
  * adds the test specified above
  * adds a class-wide volume shared by test_show_backup and
    test_show_backup_project_attribute tests (i.e. moved
    volume creation for these tests from setUp into
    resource_setup)

Change-Id: I751275633a3b6f20e67894fceb826f0f97a62b46
2 files changed