lib/tempest: Do not rely on six for image_size_in_gib
lib/tempest: Do not rely on six for image_size_in_gib

input() should work on both python versions for what we need. I
understand the concern about eval() on python2 but, in the case it's
used we should be fine, plus, python2 is being removed from OpenStack
projects.

Change-Id: I86a7c31374986f81132bc4f49aee0a76b90e6553
Signed-off-by: Lucas Alvares Gomes <ef89b923b1145c9ec2fd96eee513279016a7a963@gmail.com>

openstack-dev/devstack,nawawi/openstack,nawawi/openstack,openstack-dev/devstack