'dsidm uniquegroup members' is not implemented

[mirielka]

Issue Description
When trying to run 'dsidm uniquegroup members' command, it fails because of missing method to cover its' functionality.

Steps to Reproduce

1. Create a uniquegroup
2. Try to list its' members using 'dsidm uniquegroup members'

Expected results
List of members of the uniquegroup

Actual results

# dsidm localhost -b dc=example,dc=com uniquegroup members
Enter cn of group : uniquegroup
Error: 'UniqueGroup' object has no attribute 'list_members'