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

nova/unit.py

@@ -0,0 +1,30 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2013 IBM Corp
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+"""
+Unit constants
+"""
+
+#Byte unit constant.
+Ki = 1024
+Mi = 1024 ** 2
+Gi = 1024 ** 3
+Ti = 1024 ** 4
+Pi = 1024 ** 5
+Ei = 1024 ** 6
+Zi = 1024 ** 7
+Yi = 1024 ** 8