Add reset-state function for backups

Since there are volume reset-state function and snapshot reset-state
function, backup also needs reset-state as well.
When creating or restoring backup, it may leave the backup stuck in
creating or restoring status due to database down or rabbitmq down, etc.
Currently we could only solve these problems by restarting cinder-backup
service.

This BP is to provide another means for administrators to solve these
problems by calling backup reset state API, instead of directly
restarting cinder-backup service.

DocImpact: Support reset backup status

blueprint support-reset-state-for-backup

Change-Id: Icd677a0b48083894bcd969d5199fa91b307080de
Merge "Add reset-state function for backups"