Add byte unit constants

A lots of numeric expressions are needed to caculate byte number
across Nova project, such as 1024*1024 , 1024**3 etc. This patch
add constants and will replace these numeric expressions in
following patches. That makes code more readable.Use these constants
like: root_disk_gb = 5 * unit.Gi

blueprint byte-unit-clean

Change-Id: Ic9aad949733d375aaa695d4292ff1b171d1def78