verify-python-fuelclient job fails on import

[Dmitry Kaigarodеsev]

https://ci.fuel-infra.org/job/verify-python-fuelclient/1295/console

Traceback (most recent call last):
  File "manage.py", line 385, in <module>
    action(params) if action else parser.print_help()
  File "manage.py", line 256, in action_syncdb
    syncdb()
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/db/sqlalchemy/__init__.py", line 178, in syncdb
    do_upgrade_head()
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/db/migration/__init__.py", line 107, in do_upgrade_head
    do_upgrade_head_extensions()
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/db/migration/__init__.py", line 95, in do_upgrade_head_extensions
    for extension in get_all_extensions():
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/extensions/base.py", line 27, in get_all_extensions
    from nailgun.extensions.cluster_upgrade.extension \
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/extensions/cluster_upgrade/extension.py", line 21, in <module>
    from . import handlers
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/extensions/cluster_upgrade/handlers.py", line 19, in <module>
    from nailgun.api.v1.handlers import base
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/api/v1/handlers/__init__.py", line 15, in <module>
    from nailgun.api.v1.handlers.base import forbid_client_caching
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/api/v1/handlers/base.py", line 28, in <module>
    from nailgun.api.v1.validators.graph import GraphTasksValidator
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/api/v1/validators/graph.py", line 22, in <module>
    from nailgun.orchestrator import deployment_graph
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/nailgun/orchestrator/deployment_graph.py", line 23, in <module>
    import networkx as nx
  File "/home/jenkins/workspace/verify-python-fuelclient/fuel-web/nailgun/.tox/venv/lib/python2.6/site-packages/networkx/__init__.py", line 39, in <module>
    raise ImportError(m % sys.version_info[:2])
ImportError: Python 2.7 or later is required for NetworkX (2.6 detected).
ERROR: InvocationError: '/bin/bash /home/jenkins/workspace/verify-python-fuelclient/tools/prepare_nailgun.sh'

[Aleksandra Fedorova (bookwar)]

Old VENV's were cleaned from ci slaves

[Fuel Devops McRobotson (fuel-devops-robot)]

Reviewed: https://review.fuel-infra.org/16479
Submitter: Dmitry Kaigarodsev <email address hidden>
Branch: master

Commit: 1fa1ba4c22865216ce4f432633ff8494cffd191f
Author: Anton Tcitlionok <email address hidden>
Date: Tue Feb 2 15:20:46 2016

Clean VENV in verify-python-fuelclient job

VENV is never cleaned before build, which sometimes results in old
virtualenv package being used by tox. Latest virtualenv version uses
pip 8.0.2 and most Fuel CI Jenkins slaves have virtualenv 13.1.2 (uses
pip 7.1.2) installed. This brings inconsistency in tests, because
different pip versions can affect tests' results.

Related-Bug: #1540349

Change-Id: I7389b9d37410ecdf0db22b355cb84747a182af6f