List backups avilable for restore

What does this MR do?

List the available backup files that can be used for a restore task.

Why was this MR needed?

Save users from figuring out where the backup directory is and what all backups are available. In short, something to help our users.

Screenshots (if relevant)

root@gitlab:/# gitlab-rake gitlab:backup:restore                                        
Found more than one backup: 
 1516023318_2018_01_15_10.2.5
 1516023234_2018_01_15_10.2.5
 1516026127_2018_01_15_10.4.0-rc3
Please specify which one you want to restore:
rake gitlab:backup:restore BACKUP=timestamp_of_backup

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered