Add heat_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: I9aea9f42c476ff3c6f2355a0afb21be4eea57b69

defaults/main.yml

@@ -226,6 +226,9 @@ heat_pip_packages:
   - python-troveclient
   - uwsgi
 
+# Specific pip packages provided by the user
+heat_user_pip_packages: []
+
 heat_optional_oslomsg_amqp1_pip_packages:
   - oslo.messaging[amqp1]
 

tasks/heat_install_source.yml

@@ -43,7 +43,7 @@
     venv_install_distro_package_list: "{{ heat_distro_packages }}"
     venv_pip_install_args: "{{ heat_pip_install_args }}"
     venv_pip_packages: >-
-      {{ heat_pip_packages +
+      {{ heat_pip_packages | union(heat_user_pip_packages) +
          (heat_oslomsg_amqp1_enabled | bool) | ternary(heat_optional_oslomsg_amqp1_pip_packages, []) }}
     venv_facts_when_changed:
       - section: "heat"