Implements SI/IEC unit system conversion to bytes

The issues with utils.to_bytes are

1) Ignores measure of unit and treats b and B both as Byte
2) Cannot handle SI unit system which base is 1000 (IEC=1024)
3) The name "to_bytes" didn't do a good job representing what the function does.
   Disscussed here,
       https://review.openstack.org/#/c/32093/2/openstack/common/strutils.py

This patch implements "string_to_bytes". The name comes from
"string_to_bool" in cinder/utils.py. Its merits are,

1) Handles bit and byte units
2) Handles SI and IEC unit systems
3) Has a much more intuitive function name

Closes-bug: #1189635
Closes-bug: #1193765

Change-Id: I5f1ce56f447179d1ef6ded7c956983151f5a8f18
Merge "Implements SI/IEC unit system conversion to bytes"