ci-workers: Variable overrides and process tweaks for CentOS 7

Part of #2510

contrib/ci-workers/tasks/install-pip.yml

@@ -0,0 +1,8 @@
+---
+- name: Fetch pip installer
+  get_url:
+    url: https://bootstrap.pypa.io/get-pip.py
+    dest: /tmp/get-pip.py
+
+- name: Install pip
+  command: "{{ ansible_python.executable }} /tmp/get-pip.py"

contrib/ci-workers/unix.yml

@@ -84,6 +84,10 @@
         state: present
       with_items: "{{ package_deps }}"
 
+    - name: Install pip [CentOS]
+      include: tasks/install-pip.yml
+      when: ansible_distribution == 'CentOS'
+
     - name: Install required Python modules
       pip:
         name: "{{ item }}"

contrib/ci-workers/vars/CentOS.yml

@@ -0,0 +1,13 @@
+---
+buildbot_deps: [] # Empty to remove python-pip
+build_deps:
+  - bzip2
+  - gcc
+  - gcc-c++
+  - make
+  - patch
+dist_deps:
+  - pkgconfig # Required until b556beda264308e040f8d88aca4f2f386a0183d9 is pulled in
+  - python-devel
+  - redhat-rpm-config
+pip_bin_dir: /usr/bin