fix compute units in measurement doc

related to bug#1165123 and patch (https://review.openstack.org/#/c/26462)

memory: B to MB
disk.root.size: B to GB
disk.ephemeral.size: B to GB

Change-Id: I59599c2cb7df02acc0d4349a8591a2c1bda4f9ec
Merge "fix compute units in measurement doc"