ipa-server-install: fix zonemgr argument validator
    
    Fix `ERROR 'str' object has no attribute 'decode'` when --zonemgr is
    passed to ipa-server-install.
    
    Solution copied from commit 75d26e1f0121f875bdb017b0636c02a6f5660e8a,
    function `ipaserver.install.bindinstance.zonemgr_callback` duplicates
    the behavior of the method affected by this patch.
    
    Issue: https://pagure.io/freeipa/issue/7612
    
    Signed-off-by: Armando Neto <abiagion@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>