Fixed tests failures when SKIP_ALL_TESTS_FOR_PLUGIN=True

Moved skipif statement to be called before SetUp function in tests for IDH2
and VANILLA_TWO plugins to avoid failures caused by call to
get_image_id_and_ssh_username function in SetUp function.
The fix makes test skipped instead of being failed.

Co-Authored-By: Sergey Reshetnyak <sreshetniak@mirantis.com>

Fixes bug: #1301273

Change-Id: I5e2f67b7dd48c268f62fe94374e96a61e85aff18
Merge "Fixed tests failures when SKIP_ALL_TESTS_FOR_PLUGIN=True"