Discover correct Python package prefix

Based on whether the Ansible Python interpreter is Python 2 (the
default on Ubuntu Xenial), or Python 3 (the default on Ubuntu Bionic),
Ansible modules that rely on extra Python modules require either
"python-" or "python3-" Ubuntu packages to function.

Make sure we install the right one.

Note that on any Ubuntu Bionic system that has events or async tasks
enabled, and thus installs RabbitMQ, the Bionic rabbitmq-server
package's dependency on python (that is, the Python 2 interpreter)
will effectively pull Python 2 into that system, and many Ansible
versions will use that interpreter by default, once it is installed.

References:
https://docs.ansible.com/ansible/2.8/reference_appendices/interpreter_discovery.html
https://packages.ubuntu.com/bionic/rabbitmq-server

roles/taiga-back/tasks/main.yml

@@ -25,13 +25,11 @@
     - libxml2-dev
     - libxslt-dev
     - libzmq3-dev
-    - python-dev
-    - python-pip
     - python3
     - python3-dev
     - python3-pip
+    - "{{ _taiga_python_package_prefix }}-virtualenv"
     - tmux
-    - virtualenvwrapper
     - zlib1g-dev
   tags:
     - install

roles/taiga-back/tasks/postgresql.yml

@@ -9,7 +9,7 @@
     - postgresql-contrib
     - postgresql-doc
     - postgresql-server-dev-all
-    - python-psycopg2
+    - "{{ _taiga_python_package_prefix }}-psycopg2"
   tags:
     - install
     - back-install

roles/taiga-back/tasks/saml-selfsign.yml

@@ -2,7 +2,7 @@
   become: true
   become_user: root
   apt:
-    name: python-openssl
+    name: "{{ _taiga_python_package_prefix }}-openssl"
     state: "{{ taiga_upgrade | bool | ternary('latest', 'present') }}"
   tags:
     - install

roles/taiga-webserver/tasks/ssl-selfsign.yml

@@ -2,7 +2,7 @@
   become: true
   become_user: root
   apt:
-    name: python-openssl
+    name: "{{ _taiga_python_package_prefix }}-openssl"
     state: "{{ taiga_upgrade | bool | ternary('latest', 'present') }}"
   tags:
     - install

roles/taiga/tasks/circus.yml

@@ -6,7 +6,7 @@
     state: "{{ taiga_upgrade | bool | ternary('latest', 'present') }}"
   with_items:
     - circus
-    - python-setuptools
+    - "{{ _taiga_python_package_prefix }}-setuptools"
   tags:
     - install
     - front-install

roles/taiga/tasks/main.yml

@@ -27,6 +27,13 @@
   tags:
     - always
 
+- name: set facts related to Python versioning
+  set_fact:
+    _taiga_python_major_version: "{{ ansible_python['version']['major'] }}"
+    _taiga_python_package_prefix: "{{ (ansible_python['version']['major'] == 3) | ternary('python3', 'python') }}"
+  tags:
+    - always
+
 - name: set facts identifying node roles
   set_fact:
     _taiga_is_back_node: "{{ 'taiga-back' in group_names }}"