GCE: migrate SCT builders to qa vpc

• cloned public-jenkins-builder3 to public-jenkins-builder3-qavpc
• registered public-jenkins-builder3-qavpc to jenkins as gce3-qavpc
• switch job/scylla-private-repo-test to use gce3-qavpc (the job runs well)
• We can disable gce3-qavpc, and copy its ip to gce3, when there is no task on gce3.

Builder environment:

sudo yum install gcc git screen 
sudo yum install libvirt libvirt-devel pkgconfig gdb-gdbserver git -y
curl https://raw.githubusercontent.com/avocado-framework/avocado/36lts/requirements.txt -o avocado_requirements.txt
sed -ie 's/libvirt-python/#libvirt-python/' avocado_requirements.txt
sudo pip install -r avocado_requirements.txt

curl https://raw.githubusercontent.com/scylladb/scylla-cluster-tests/master/requirements.txt -o sct_requirements.txt
sed -ie 's/libvirt-python/#libvirt-python/' sct_requirements.txt
sudo pip install -r sct_requirements.txt