(#11042) indirect via the property to convert group names to numbers

The puppet documentation for managing users says of the attribute gid
"Can be specified numerically or by name". The ldap provider does not
support specifying by gid by name as (at least on RedHat 389 Directory
Server), it generates a syntax error. Converting groupnames to numeric
group id resolves the issue.

Reviewed-by: Jeff McCune <jeff@puppetlabs.com>