Disabled xunit support in vdsm_master_unit-tests_merged

[Nir Soffer]

Hi infra,

I disable nose xunit support in this job:
http://jenkins.ovirt.org/job/vdsm_master_unit-tests_merged/

The xunit plugin trigger a segfault in Python during tests, which cause most runs
to fail.

Changes:

1. disable xunit plugin:

# xunit cause segfault in python - disabled until it is fixed.
# export NOSE_WITH_XUNIT=1 

2. Remove the post job collection of nosetests.xml

Thanks,
Nir