Stop to use the __future__ module.

The __future__ module [1] was used in this context to ensure compatibility
between python 2 and python 3.

We previously dropped the support of python 2.7 [2] and now we only support
python 3 so we don't need to continue to use this module and the imports
listed below.

Imports commonly used and their related PEPs:
- `division` is related to PEP 238 [3]
- `print_function` is related to PEP 3105 [4]
- `unicode_literals` is related to PEP 3112 [5]
- `with_statement` is related to PEP 343 [6]
- `absolute_import` is related to PEP 328 [7]

[1] https://docs.python.org/3/library/__future__.html
[2] https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html
[3] https://www.python.org/dev/peps/pep-0238
[4] https://www.python.org/dev/peps/pep-3105
[5] https://www.python.org/dev/peps/pep-3112
[6] https://www.python.org/dev/peps/pep-0343
[7] https://www.python.org/dev/peps/pep-0328

Change-Id: I9dcb02f5417c0f1d0ffce94267953a426252a56b

glance_store/_drivers/rbd.py

@@ -15,8 +15,6 @@
 
 """Storage backend for RBD
    (RADOS (Reliable Autonomic Distributed Object Store) Block Device)"""
-from __future__ import absolute_import
-from __future__ import with_statement
 
 import contextlib
 import hashlib

tools/install_venv.py

@@ -21,8 +21,6 @@
 Installation script for glance_store's development virtualenv
 """
 
-from __future__ import print_function
-
 import os
 import sys
 

tools/install_venv_common.py

@@ -22,8 +22,6 @@ environment, it should be kept strictly compatible with Python 2.6.
 Synced in from openstack-common
 """
 
-from __future__ import print_function
-
 import optparse
 import os
 import subprocess