Fix the upload test cases mechanism to work with pytest tests and to be more stable

[Alexander Kurenyshev]

As we can see here [1] an updating test case fields in test rail fails with:
Traceback (most recent call last):
  File "fuelweb_test/testrail/upload_cases_description.py", line 497, in <module>
    main()
  File "fuelweb_test/testrail/upload_cases_description.py", line 494, in main
    check_all_sections=not options.check_one_section)
  File "fuelweb_test/testrail/upload_cases_description.py", line 172, in upload_tests_descriptions
    fields_to_update = _get_fields_to_update(test_case, testrail_case)
  File "fuelweb_test/testrail/upload_cases_description.py", line 397, in _get_fields_to_update
    if test_case[field] and \
KeyError: u'custom_job_settings'

The only one reason for it could be pytest tests which haven't field `custom_job_settings` in their test_case structure [2]. Need to add this field to fix the error above.

Second error is related to the api call to `injectedEnvVars`. For example [3]. For some reason we failed on the step with a decoding json. To make code more stable we should avoid this errors.

[1] https://product-ci.infra.mirantis.net/job/10.0.system_test.test-reports/8041/console
[2] https://github.com/openstack/fuel-qa/blob/0f50e7606a5b717b480f937d81e5663068c3bc9d/fuelweb_test/testrail/upload_cases_description.py#L91-L101
[3] https://product-ci.infra.mirantis.net/job/10.0.swarm.failures_report/2195/console

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/357915

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/357915
Committed: https://git.openstack.org/cgit/openstack/fuel-qa/commit/?id=332d1ba30f57de65a311f04568fccc9939dac39f
Submitter: Jenkins
Branch: master

commit 332d1ba30f57de65a311f04568fccc9939dac39f
Author: Alexander Kurenyshev <email address hidden>
Date: Fri Aug 19 17:23:16 2016 +0300

    Fix upload test rail cases mechanism

    1. Fix an error with abcent of `custom_job_settings` field
    for the pytest tests.
    2. Add try/except block for a getting injected vars from several jobs

    See an attached bug for more info.

    Change-Id: I0aa95fa602a8965376fb20b3a6c2f1b3d9a2ebd7
    Closes-Bug:1614985