Allow overriding of Bazel installer checksum

Checksum retrieval from Github doesn't work when Artifactory is used as
a Github mirror when the installer is not already cached.

Allow setting the Bazel installer checksum as a variable to make the
role work in such cases.

See also https://www.jfrog.com/jira/browse/RTFACT-22923

Change-Id: Icc3480420895b9052a4f1c133659a31fff0723be

roles/ensure-bazel/README.rst

@@ -11,3 +11,9 @@ Download and install Bazel, if the specified version is not already present.
    :default: 'https://github.com/bazelbuild/bazel/releases/download'
 
    The base URL to use when downloading Bazel releases.
+
+.. zuul:rolevar:: bazel_installer_checksum
+   :default: None
+
+   The Bazel installer SHA256 checksum. If not provided, the checksum will be
+   retrieved from Github.

roles/ensure-bazel/defaults/main.yaml

@@ -1,4 +1,5 @@
 ---
 bazel_version: '3.1.0'
 bazel_release_url: 'https://github.com/bazelbuild/bazel/releases/download'
+bazel_installer_checksum:
 install_bazel_if_missing: true

roles/ensure-bazel/tasks/install-bazel.yaml

@@ -3,22 +3,29 @@
     state: directory
   register: bazel_installer_tempdir
 
-- name: Get installer checksum
-  uri:
-    url: "{{ bazel_release_url }}/{{ bazel_version }}/bazel-{{ bazel_version }}-installer-linux-x86_64.sh.sha256"
-    return_content: true
-  register: bazel_installer_checksum
+- name: Fetch checksum from Github
+  when: not bazel_installer_checksum
+  block:
+    - name: Get installer checksum
+      uri:
+        url: "{{ bazel_release_url }}/{{ bazel_version }}/bazel-{{ bazel_version }}-installer-linux-x86_64.sh.sha256"
+        return_content: true
+      register: zj_checksum_response
+
+    - name: Set installer checksum
+      set_fact:
+        bazel_installer_checksum: "{{ zj_checksum_response.content.split(' ')[0] }}"
 
 - name: Display checksum
   debug:
-    msg: "Checksum is {{ bazel_installer_checksum.content.split(' ')[0] }}"
+    msg: "Checksum is {{ bazel_installer_checksum }}"
 
 - name: Download bazel installer
   get_url:
     url: "{{ bazel_release_url }}/{{ bazel_version }}/bazel-{{ bazel_version }}-installer-linux-x86_64.sh"
     dest: "{{ bazel_installer_tempdir.path }}/bazel-{{ bazel_version }}-installer-linux-x86_64.sh"
     mode: 0755
-    checksum: "sha256:{{ bazel_installer_checksum.content.split(' ')[0] }}"
+    checksum: "sha256:{{ bazel_installer_checksum }}"
 
 - name: Display distribution
   debug: