[CI] libvirt-gitlab-executor project is now part of the libvirt namespace

[Erik Skultety]

This is just a small announcement that the $subj project which was created in
order to provision customized VMs as throwaway worker nodes for our functional
test suite utilizing GitLab's custom executor feature was transferred to the
libvirt's GitLab namespace.
Anything that relates to both infrastructure and executing the functional test
suite in a customized environment should go to this project.

Regards,
Erik