Currently on CentOS7, while deploying nova, it gives libvirt
module import error. By adding venv_packages_to_symlink in
nova_source_install will symlink the packages defined in
nova_compute_kvm_packages_to_symlink and fixes the issue.
It removes nova_compute_kvm_install.yml to use python_venv_build
role.
Change-Id: I20a181c6c4901b3cc846a9f665e5def98aac5059