Add zuul_user_shell variable

Give the ability for a user to override the default shell for zuul user
account.

Change-Id: I30cb9a13f4eaf799abb627a99780aef84eaec286
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

defaults/main.yaml

@@ -23,6 +23,7 @@ zuul_task_manager:
 zuul_user_name: zuul
 zuul_user_group: zuul
 zuul_user_home: /var/lib/zuul
+# zuul_user_shell:
 
 zuul_file_main_yaml_dest: /etc/zuul/main.yaml
 zuul_file_main_yaml_group: "{{ zuul_user_group }}"

tasks/pre.yaml

@@ -24,3 +24,4 @@
     home: "{{ zuul_user_home }}"
     group: "{{ zuul_user_group }}"
     name: "{{ zuul_user_name }}"
+    shell: "{{ zuul_user_shell | default(omit) }}"