Add zuul_git_update setting

It is possible we do not want to update git repos or already have git
repos on disk.

Change-Id: I5095cdc3c0d25027164d98f36c8ecbeba426a6cb
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

defaults/main.yaml

@@ -64,6 +64,7 @@ zuul_file_zuul_conf_src: etc/zuul/zuul.conf
 zuul_git_dest: /opt/ansible-role-zuul/git/openstack-infra/zuul
 zuul_git_uri: https://git.openstack.org/openstack-infra/zuul
 zuul_git_version: feature/zuulv3
+# zuul_git_update:
 
 zuul_install_method: git
 

tasks/install/git.yaml

@@ -17,6 +17,7 @@
   git:
     dest: "{{ zuul_git_dest }}"
     repo: "{{ zuul_git_uri }}"
+    update: "{{ zuul_git_update|default(omit) }}"
     version: "{{ zuul_git_version }}"
 
 - name: Set zuul_pip_name to local git repo.