Add keystone_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: If2a01309f37c585cdd6a7d8d1f6d893e0707626a

defaults/main.yml

@@ -448,6 +448,9 @@ keystone_pip_packages:
   - uWSGI
   - pyngus
 
+# Specific pip packages provided by the user
+keystone_user_pip_packages: []
+
 # optional pip packages
 keystone_optional_oslomsg_amqp1_pip_packages:
   - oslo.messaging[amqp1]

tasks/keystone_install_source.yml

@@ -43,7 +43,7 @@
     venv_install_destination_path: "{{ keystone_bin | dirname }}"
     venv_pip_install_args: "{{ keystone_pip_install_args }}"
     venv_pip_packages: >-
-      {{ keystone_pip_packages +
+      {{ keystone_pip_packages | union(keystone_user_pip_packages) +
          (keystone_oslomsg_amqp1_enabled | bool) | ternary(keystone_optional_oslomsg_amqp1_pip_packages, []) }}
     venv_facts_when_changed:
       - section: "keystone"