Add ability to set the user's shell, using a sane, per-platform default

Fix case insensitivity issue with platform included filenames

Change-Id: I434a89b2b0534c7883c35e4e5536346d41f90bf6
diff --git a/tasks/main.yml b/tasks/main.yml
index befc0dc..8ba9c7c 100644
--- a/tasks/main.yml
+++ b/tasks/main.yml
@@ -22,6 +22,7 @@
     password: "{{ item[users_os_pw_type] }}"
     home: "{{ item.homedir | default(omit) }}"
     system: "{{ item.system | default(false) }}"
+    shell: "{{ item.shell | default(users_default_shell) }}"
   with_items: "{{ userlist }}"
 
 - name: Add user to sudo-capable group if they're a sudoer