Drop private argument for include/import role

Since ansible 2.8 dropped private argument is not supported anymore:
https://github.com/ansible/ansible/issues/45038

Change-Id: I13a46af91e3ccd754e71f0ed7c49a187d488f090

tasks/glance_install.yml

@@ -130,7 +130,6 @@
 - name: Install the python venv
   import_role:
     name: "python_venv_build"
-    private: yes
   vars:
     venv_build_constraints: "{{ glance_git_constraints }}"
     venv_build_distro_package_list: "{{ glance_devel_distro_packages }}"