Add glance_user_pip_packages variable

With this variable, users would be able to extend
the list of pip packages in case of needing an
extra pip package.

Currently if we need an extra pip package we need
to override the existing list.

Change-Id: I59bd71a581b23574323db55f0f16194e7788e22e

defaults/main.yml

@@ -263,6 +263,9 @@ glance_pip_packages:
   - uwsgi
   - warlock
 
+# Specific pip packages provided by the user
+glance_user_pip_packages: []
+
 glance_optional_oslomsg_amqp1_pip_packages:
   - oslo.messaging[amqp1]
 

tasks/glance_install_source.yml

@@ -43,7 +43,8 @@
     venv_install_destination_path: "{{ glance_bin | dirname }}"
     venv_install_distro_package_list: "{{ glance_distro_packages }}"
     venv_pip_install_args: "{{ glance_pip_install_args }}"
-    venv_pip_packages: "{{ (glance_oslomsg_amqp1_enabled | bool) | ternary(glance_pip_packages + glance_optional_oslomsg_amqp1_pip_packages, glance_pip_packages) }}"
+    venv_pip_packages: "{{ glance_pip_packages | union(glance_user_pip_packages) +
+    (glance_oslomsg_amqp1_enabled | bool) | ternary(glance_optional_oslomsg_amqp1_pip_packages, []) }}"
     venv_facts_when_changed:
       - section: "glance"
         option: "need_service_restart"