ipagroup: Handle ensuring groups with mixed types without IPA fix 6741

Ensuring (adding) several groups with mixed types external, nonposix
and posix require to have a fix in IPA:

    FreeIPA issue: https://pagure.io/freeipa/issue/9349
    FreeIPA fix: https://github.com/freeipa/freeipa/pull/6741

The simple solution is to switch to client context for ensuring several
groups simply if the user was not explicitly asking for the server context
no matter if mixed types are used.