[system-tests] Update all SkipTest() with corresponding error message

[Vladimir Khlyunev]

Now we are using SkipTest without any error message - so it's difficult to figure out why the test was skipped (previous test in dependency tree failed or the env configured incorrectly). We should add error message for all SkipTest() calls for improving readability of our logs.

For example, in [0] we are skipping the test because the env configured incorrectly;
in [1] we suppress the error message (even if exists it will be removed);
in [2] we check 'should we run the test' and if not - we are skipping it without any message.

[0] https://github.com/openstack/fuel-qa/blob/42aadc4c537a385f5c0a3d34b86df63f0912a6ee/fuelweb_test/tests/test_multiple_networks.py#L60
[1] https://github.com/openstack/fuel-qa/blob/a8c4492e12ce58badffa5e90caab6672c6c0e732/fuelweb_test/helpers/decorators.py#L82
[2] https://github.com/openstack/fuel-qa/blob/7ce54c7d16d8bf1297ac08accf00d08823b8130f/fuelweb_test/tests/base_test_case.py#L57

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.openstack.org/244798

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/244798
Committed: https://git.openstack.org/cgit/openstack/fuel-qa/commit/?id=c6e10735e5b8078342414891d8500ad56b63388a
Submitter: Jenkins
Branch: master

commit c6e10735e5b8078342414891d8500ad56b63388a
Author: Artem Panchenko <email address hidden>
Date: Thu Nov 12 19:59:14 2015 +0200

    Add skip message to result comments

    Change-Id: I38e12f4735bb83977e19a1eee75fe905fce0a3fc
    Related-bug: #1515601

[Tatyanka (tatyana-leontovich)]

move to 9.0 according to we do not fix medium issues in 8.0 anymore

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/310454

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/310454
Committed: https://git.openstack.org/cgit/openstack/fuel-qa/commit/?id=75c35894a25623a947451e2b988240d8024a7028
Submitter: Jenkins
Branch: master

commit 75c35894a25623a947451e2b988240d8024a7028
Author: Maksym Strukov <email address hidden>
Date: Wed Apr 27 15:49:02 2016 +0300

    Updating SkipTest() with error messages

    Change-Id: I1d11bd25f9d40c2f9289bd4c68f00aca04ff4733
    Closes-Bug: #1515601