Allow repo host packages to be upgraded before repo build

To cater for a situation where the repo build packages are changing,
but the repo does not yet have the package built, we need to ensure
that the install task for it can fetch from pypi (or the designated
pypi mirror). To do this we try the local repo first, then fall back
to using the designated default index which defaults to pypi.

Change-Id: Iba7e9939250b24dc64fdd1883f963d0253ddad7e

tasks/repo_build_prepare.yml

@@ -89,17 +89,40 @@
       (repo_build_wheel_rebuild | bool) or
       (repo_build_venv_rebuild | bool)
 
-- name: Install pip packages
-  pip:
-    name: "{{ repo_pip_packages }}"
-    state: "{{ repo_build_pip_package_state }}"
-    extra_args: "--constraint {{ repo_build_release_path }}/requirements_constraints.txt {{ pip_install_options }}"
-  register: install_packages
-  until: install_packages|success
-  retries: 5
-  delay: 5
+# Note(odyssey4me):
+# To cater for a situation where the pip packages are changing, but the repo
+# does not yet have the package built, we need to ensure that this task can
+# fetch from pypi. To do this we try the local repo first, then fall back to
+# using pypi.
+- name: Try installing from the repo first, then fall back to using pypi
   tags:
     - repo-build-install
+  block:
+    - name: Install pip packages (from repo)
+      pip:
+        name: "{{ repo_pip_packages }}"
+        state: "{{ repo_build_pip_package_state }}"
+        extra_args: >-
+          --constraint {{ repo_build_release_path }}/requirements_constraints.txt
+          {{ pip_install_options }}
+      register: install_packages
+      until: install_packages | success
+      retries: 5
+      delay: 5
+  rescue:
+    - name: Install pip packages (from pypi mirror)
+      pip:
+        name: "{{ repo_pip_packages }}"
+        state: "{{ repo_build_pip_package_state }}"
+        extra_args: >-
+          --index-url {{ repo_build_pip_default_index }}
+          --trusted-host {{ repo_build_pip_default_index | netloc_no_port }}
+          --constraint {{ repo_build_release_path }}/requirements_constraints.txt
+          {{ pip_install_options }}
+      register: install_packages
+      until: install_packages | success
+      retries: 5
+      delay: 5
 
 - name: Create repo log directory
   file: