"raw_input()" deprecated in Python3

Built-in function "raw_input()" is deprecated in favor of "input()" [1].

[1] https://www.python.org/dev/peps/pep-3111/

Change-Id: I31c4c59373a2ad04987da3daffb3eed50916a6db
Closes-Bug: #1823321
diff --git a/lib/tempest b/lib/tempest
index 8c6fa01..95b138c 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -107,7 +107,7 @@
 function image_size_in_gib {
     local size
     size=$(openstack image show $1 -c size -f value)
-    echo $size | python -c "import math; print int(math.ceil(float(int(raw_input()) / 1024.0 ** 3)))"
+    echo $size | python -c "import math; import six; print(int(math.ceil(float(int(six.moves.input()) / 1024.0 ** 3))))"
 }
 
 # configure_tempest() - Set config files, create data dirs, etc