Fix backup execution workflow

This patch ensures that _run_post_backup() always runs after
the backup operation has completed (post-backup). Fixes issue
wherein any cleanup occuring in _run_post_backup would race
with commands running during backup (potentially unpredictable
dangerous situation).

Change-Id: I279c9a85415ac6ec2ddf8e564a056a3415690f68
Closes-Bug: #1340864
Merge "Fix backup execution workflow"