Adding GET API to backup

Get backup by ID.

Change-Id: I0a6422ab12449212d144d9fdec1a4c7b282bed85
BP: blueprints.launchpad.net/reddwarf/+spec/consistent-snapshots">https://blueprints.launchpad.net/reddwarf/+spec/consistent-snapshots
Merge "Adding GET API to backup"