Temporarily disable VNC tests

Temporarily disable VNC tests

At least Compass and Fuel configure Spice as console [1][2].
It disables vnc testing as it could fail for endusers as well.

[1] https://artifacts.opnfv.org/logs/functest/huawei-virtual9/2018-10-24_04-54-25/tempest_full/tempest-report.html
[2] https://artifacts.opnfv.org/logs/functest/lf-pod2/gambia/2018-10-25_05-57-54/tempest_full/tempest-report.html

Conflicts:
    functest/opnfv_tests/openstack/tempest/conf_utils.py

Change-Id: I2a1ecf345cb81170c1b1a3e49f8f3a58602fc3f0
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit df9d5c5d3b80b613130de59a1810f7fce9cbbc0d)