verify-python-fuelclient-on-fuel-web CI job fails sometimes

[Aleksey Kasatkin]

Test run fail on the following:

Setting up database.
INFO [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO [alembic.runtime.migration] Will assume transactional DDL.
INFO [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO [alembic.runtime.migration] Will assume transactional DDL.
INFO [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO [alembic.runtime.migration] Will assume transactional DDL.
Traceback (most recent call last):
  File "manage.py", line 382, in <module>
    action(params) if action else parser.print_help()
  File "manage.py", line 244, in action_loaddefault
    fixman.load_fake_deployment_tasks()
  File "/home/jenkins/workspace/verify-python-fuelclient-on-fuel-web/nailgun/nailgun/db/sqlalchemy/fixman.py", line 56, in load_fake_deployment_tasks
    {'tasks': deployment_tasks}, rel)
  File "/home/jenkins/workspace/verify-python-fuelclient-on-fuel-web/nailgun/nailgun/objects/deployment_graph.py", line 188, in create_for_model
    'Graph of given type already exists for this model.')
nailgun.errors.validation.AlreadyExists: Graph of given type already exists for this model.
ERROR: InvocationError: '/bin/bash /home/jenkins/workspace/verify-python-fuelclient-on-fuel-web/python-fuelclient/tools/prepare_nailgun.sh'

E.g.

https://ci.fuel-infra.org/job/verify-python-fuelclient-on-fuel-web/4445/console
https://ci.fuel-infra.org/job/verify-python-fuelclient-on-fuel-web/4450/console