Making main user's shell configurable

roles/common/tasks/users.yml

 - name: Create main user account
-  user: name={{ main_user_name }} state=present shell=/usr/bin/zsh groups=sudo,fuse
+  user: name={{ main_user_name }} state=present shell={{ main_user_shell }} groups=sudo,fuse

vars/defaults.yml

 # domain: (required)
 # main_user_name: (required)
 admin_email: "{{ main_user_name }}@{{ domain }}"
+main_user_shell: "/bin/bash"
 # encfs_password: (required)
 friendly_networks:
   - ""