create-env job fails if nothing to clean

[Dmitry Kaigarodеsev]

Since we're using 'set -ex'[0] bash option in update script - it fails[1] if cannot grep env names[2] over pipe

++ VIRTUAL_ENV=/home/jenkins/venv-nailgun-tests-2.9
++ export VIRTUAL_ENV
++ _OLD_VIRTUAL_PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
++ PATH=/home/jenkins/venv-nailgun-tests-2.9/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
++ export PATH
++ '[' -n '' ']'
++ '[' -z '' ']'
++ _OLD_VIRTUAL_PS1=
++ '[' x '!=' x ']'
+++ basename /home/jenkins/venv-nailgun-tests-2.9
++ '[' venv-nailgun-tests-2.9 = __ ']'
+++ basename /home/jenkins/venv-nailgun-tests-2.9
++ PS1='(venv-nailgun-tests-2.9)'
++ export PS1
++ alias 'pydoc=python -m pydoc'
++ '[' -n /bin/bash -o -n '' ']'
++ hash -r
++ dos.py list
++ grep '^env_master'
+ ENV_NAMES=
Build step 'Execute shell' marked build as failure

we need to add a check for existing`

[0] https://github.com/fuel-infra/jenkins-jobs/blob/master/servers/fuel-ci/builders/update-env.sh#L3
[1] https://ci.fuel-infra.org/job/devops.master.create-env/951/console
[2] https://github.com/fuel-infra/jenkins-jobs/blob/master/servers/fuel-ci/builders/update-env.sh#L19

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

Fix proposed to branch: master
Change author: Dmitry Kaigarodsev <email address hidden>
Review: https://review.fuel-infra.org/19494

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

Reviewed: https://review.fuel-infra.org/19494
Submitter: Dmitry Kaigarodsev <email address hidden>
Branch: master

Commit: c3f0cc4480be400d5445e764b9ae207196df1c95
Author: Dmitry Kaigarodsev <email address hidden>
Date: Tue Apr 12 06:56:35 2016

Fix for 'devops.create-env' script:

* added check 'if virtual environments are
  exists' before venv cleanup

Change-Id: I8f947ce218d335fb6f57a358288d736d38659828
Closes-Bug: #1569034