Use a dict instead of a list for users

My main motivation for making this change is so that you can do things like:

`{{ salt['pillar.get']('mysql:user:db_user:password', 'pass') }}`
`{{ salt['pillar.get']('mysql:user:db_user:host', 'localhost') }}`

Which otherwise I wasn't able to figure out in a clean way. Also, dict seems more consistent with some of the other formulas I've been using so far:
https://github.com/saltstack-formulas/users-formula/blob/master/pillar.example
https://github.com/saltstack-formulas/apache-formula/blob/master/pillar.example

Edit: and if I'm not mistaken this also means you can easily separate your users into separate pillar files.