Documentation for keystone_user

Updated the module with proper documentation.
hide magic variables used bin/ansible output

'verbose_override' and 'verbose_always'
Merge pull request #3027 from sergevanginderachter/verbose_always_tag_cli

hide magic variables used bin/ansible output
Merge pull request #3025 from lorin/keystone-docs

Documentation for keystone_user
Support JSON in --extra-vars.

If --extra-vars starts with either a '{' or a '[', it will be assumed
we are dealing with JSON and parse the data as such.