Add cinder_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: If7637ac750330822e7f5e3cae6a78315bc25031b

defaults/main.yml

@@ -304,6 +304,9 @@ cinder_pip_packages:
   - systemd-python
   - uwsgi
 
+# Specific pip packages provided by the user
+cinder_user_pip_packages: []
+
 cinder_optional_oslomsg_amqp1_pip_packages:
   - oslo.messaging[amqp1]
 

tasks/cinder_install_source.yml

@@ -43,7 +43,7 @@
     venv_install_destination_path: "{{ cinder_bin | dirname }}"
     venv_pip_install_args: "{{ cinder_pip_install_args }}"
     venv_pip_packages: >-
-      {{ cinder_pip_packages +
+      {{ cinder_pip_packages | union(cinder_user_pip_packages) +
          (cinder_oslomsg_amqp1_enabled | bool) | ternary(cinder_optional_oslomsg_amqp1_pip_packages, []) }}
     venv_facts_when_changed:
       - section: "cinder"