Expose default dotfiles state in a variable (users)

Using 'users_dotfiles_enabled_default' bool variable you can set the
default state of dotfiles on all user accounts managed by Ansible
'users' role. If False (default), dotfiles will not be configured (makes
configuration significantly faster). If True, dotfiles will be
configured by default on all accounts.

You can combine this setting with 'item.dotfiles' option on each user
account, essentially toggling the default behaviour - if default dotfile
state is True, then you can configure user account with option
'dotfiles: False' to disable dotfiles on that particular account.
Enable dotfiles by default for all Vagrant users

I don't care, what they're going to say,
Let the storm rage on,
The dotfiles never bothered me anyway.
Merge pull request #22 from drybjed/master

Changes in dotfile configuration in 'users' role