Use upper constraints when installing virtualbmc

The lack of UC means that pip is attempting to install libvirt-python
4.1.0, which is failing on Ubuntu due to
https://bugs.launchpad.net/openstack-requirements/+bug/1753539.

Currently upper-constraints limits libvirt-python to 4.0.0, which does
not have this issue.

Change-Id: I97e055aa1d4ebdd85693eeb8228b58a6bc9bad9a
Related-Bug: #1753539

playbooks/roles/bifrost-create-vm-nodes/defaults/main.yml

@@ -40,3 +40,5 @@ bifrost_venv_dir: "{{ lookup('env', 'VENV') | default('/opt/stack/bifrost') }}"
 bifrost_venv_env:
   VIRTUAL_ENV: "{{ bifrost_venv_dir }}"
   PATH: "{{ bifrost_venv_dir }}/bin:{{ ansible_env.PATH }}" # include regular path via lookup env
+reqs_git_folder: /opt/stack/requirements
+upper_constraints_file: "{{ lookup('env', 'UPPER_CONSTRAINTS_FILE') | default(reqs_git_folder + '/upper-constraints.txt', True) }}"

playbooks/roles/bifrost-create-vm-nodes/tasks/prepare_libvirt.yml

@@ -134,3 +134,4 @@
 - name: install virtualbmc
   pip:
     name: virtualbmc
+    extra_args: "-c {{ upper_constraints_file }}"