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/vars/Debian.yml b/vars/Debian.yml
index 5bed69b..66b08c9 100644
--- a/vars/Debian.yml
+++ b/vars/Debian.yml
@@ -10,3 +10,5 @@
 users_os_sudoers_group: "sudo"
 
 users_os_pw_type: "sha512crypt"
+
+users_default_shell: "/bin/bash"