sync units.py from oslo to ceilometer

This patch syncs the units module from oslo to ceilometer.

Change-Id: Iaf3a357738e71ef37fa3dd895b25d6335bf4f544
Use the module units to refer bytes type

Change-Id: I7d3e1b93fcbd7d60593d44bb86899fb43dc69003
Merge "sync units.py from oslo to ceilometer"
Merge "Use the module units to refer bytes type"