Expose groups params of the user type in govuk::user
Expose groups params of the user type in govuk::user

This commit:
 - Exposes the groups parameter of the user resource in govuk::user
   - leaving the default groups to add a user to unchanged
 - Exposes the membership parameter of the user resource as the
   purgegroups boolean for govuk::user
   - This allows purging from groups not explicitly listed

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet