ipagroup.py

A group membership manager is a user or a group that can add members to
a group or remove members from a group.

This is related to https://pagure.io/freeipa/issue/8114

New parameters have been added to the module:
- `membermanager_user`: List of member manager users assigned to this
  group. Only usable with IPA versions 4.8.4 and up.
- `membermanager_group`: List of member manager groups assigned to this
  group. Only usable with IPA versions 4.8.4 and up.

These parameters behave like member parameters.

A new test has been added:
- tests/group/test_group_membermanager.yml