Forcing restoreAll breaks the upgrade path if backups have never been used

[rhyas]

Upgrading to 1.7 from 1.6:

INFO com.netflix.exhibitor.core.s3.S3ClientImpl Setting S3 endpoint to: https://s3.amazonaws.com [main]
v1.7.0
INFO com.netflix.exhibitor.core.activity.ActivityLog Exhibitor started [main]
INFO com.netflix.exhibitor.core.backup.BackupManager Restoring log files from backup. [main]
Exception in thread "main" java.lang.IllegalStateException: Optional.get() cannot be called on an absent value
at com.google.common.base.Absent.get(Absent.java:47)
at com.netflix.exhibitor.core.backup.BackupManager.getAvailableBackups(BackupManager.java:151)
at com.netflix.exhibitor.core.backup.BackupManager.restoreAll(BackupManager.java:202)
at com.netflix.exhibitor.core.Exhibitor.start(Exhibitor.java:180)
at com.netflix.exhibitor.application.ExhibitorMain.(ExhibitorMain.java:132)
at com.netflix.exhibitor.application.ExhibitorMain.main(ExhibitorMain.java:94)

When backups were never configured, exhibitor fails to start.

PR #350 is what seems to have caused this issue.