Make image sizes more readable for humans

By introducing the parameter --human-readable for several functions
(image-list, image-show, image-update, image-create) it's possible
to convert the size in bytes to something more readable like
9.309MB or 1.375GB.

Change-Id: I4e2654994361dcf330ed6d681dbed73388f159cb

glanceclient/common/utils.py

@@ -168,3 +168,15 @@ def integrity_iter(iter, checksum):
         raise IOError(errno.EPIPE,
                       'Corrupt image download. Checksum was %s expected %s' %
                       (md5sum, checksum))
+
+
+def make_size_human_readable(size):
+    suffix = ['B', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB']
+    base = 1024.0
+
+    index = 0
+    while size > base:
+        index = index + 1
+        size = size / base
+
+    return "%.3f%s" % (size, suffix[index])