Add BackupStatus enum field

Add BackupStatus enum field

This change adds a new enum and field, BackupStatus and
BackupStatusField, that will hold the constants for the 'status' field
of the Backup object. This enum and field are based off the base
oslo.versionedobjects enum and field. This also changes over the Backup
object to use the new field (and bumps the version so newer versions
know to enforce valid values). Finally, all uses of strings for
comparison and assignment to this field are changed over to use the
constants defined within the enum.

Change-Id: I3df71e361f2bd37417133a4c1189e37466ed0b56
Implements: bp cinder-object-fields