Add magnum_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: Iaa94479ea780aa0b79b880327b807223f66874d6

defaults/main.yml

@@ -160,6 +160,9 @@ magnum_pip_packages:
   - python-memcached
   - uwsgi
 
+# Specific pip packages provided by the user
+magnum_user_pip_packages: []
+
 magnum_optional_oslomsg_amqp1_pip_packages:
   - oslo.messaging[amqp1]
 

tasks/magnum_install.yml

@@ -42,7 +42,8 @@
     venv_install_destination_path: "{{ magnum_bin | dirname }}"
     venv_install_distro_package_list: "{{ magnum_distro_packages }}"
     venv_pip_install_args: "{{ magnum_pip_install_args }}"
-    venv_pip_packages: "{{ (magnum_oslomsg_amqp1_enabled | bool) | ternary(magnum_pip_packages + magnum_optional_oslomsg_amqp1_pip_packages, magnum_pip_packages) }}"
+    venv_pip_packages: "{{ magnum_pip_packages | union(magnum_user_pip_packages) +
+    (magnum_oslomsg_amqp1_enabled | bool) | ternary(magnum_optional_oslomsg_amqp1_pip_packages, []) }}"
     venv_facts_when_changed:
       - section: "magnum"
         option: "venv_tag"