Add neutron_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: If5c860b52af592629317def2c9c21f166a9c6ed1

tasks/neutron_install_source.yml

@@ -43,7 +43,7 @@
     venv_install_distro_package_list: "{{ neutron_package_list }}"
     venv_pip_install_args: "{{ neutron_pip_install_args }}"
     venv_pip_packages: >-
-      {{ neutron_pip_packages +
+      {{ neutron_pip_packages | union(neutron_user_pip_packages) +
          (neutron_bgp | bool) | ternary(neutron_optional_bgp_pip_packages, []) +
          (neutron_fwaas | bool) | ternary(neutron_optional_fwaas_pip_packages, []) +
          (neutron_fwaas_v2 | bool) | ternary(neutron_optional_fwaas_pip_packages, []) +

vars/main.yml

@@ -35,6 +35,9 @@ neutron_pip_packages:
   - PyMySQL
   - python-memcached
 
+# Specific pip packages provided by the user
+neutron_user_pip_packages: []
+
 neutron_optional_oslomsg_amqp1_pip_packages:
   - oslo.messaging[amqp1]