delete servers in setUpClass if ecxeption raised

this catch the exception in setUpClass of test_images_oneserver
and delete the server. Every exception should be catched
so hasn't specify what kind of exception after except.

fix bug 1158843

Change-Id: I4ea4f6b4bc97a514393c0d0cb6236dcc720fd1a0
(cherry picked from commit bf88a63c8edb08f03a96b91e76eb4630fe4a3426)