We should raise error if someone try to run group that is not discovered by test runner

[Tatyanka]

For now if we try run the test group that t is not discovered, we get just message :
+ export ENV_NAME
+ export LOGS_DIR
+ export ISO_PATH
+ [ != yes ]
+ [ no = yes ]
+ dos.py list
+ grep ^env_master_gate_ostf_update$
+ [ env_master_gate_ostf_update ]
+ dos.py erase env_master_gate_ostf_update
+ OPTS=
+ [ -n ]
+ [ -n ]
+ [ -n --group=gate_ostf_update ]
+ OPTS= --group=gate_ostf_update
+ [ no = yes ]
+ export PYTHONPATH=/home/jenkins/workspace/systest/master
+ echo /home/jenkins/workspace/systest/master
/home/jenkins/workspace/systest/master
+ python run_system_test.py run -q --nologcapture --with-xunit --group=gate_ostf_update

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK

It is better to raise error to avoid false positive test results

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/280614

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/280614
Committed: https://git.openstack.org/cgit/openstack/fuel-qa/commit/?id=e8eae3dc0e57b9a9d3b66c7cdcbcc10f560c1fef
Submitter: Jenkins
Branch: master

commit e8eae3dc0e57b9a9d3b66c7cdcbcc10f560c1fef
Author: Tatyana Leontovich <email address hidden>
Date: Tue Feb 16 13:33:55 2016 +0200

    Call system.exit if we run non existing group

    Change-Id: I2448d824a0d3f052720dc9e495b44239a699fb67
    Closes-Bug: #1545727