Fix winsync agreements creation
Due to recent addition of ID range support to DsInstance, the class
could no longer be instantiated when realm_name was passed but
ID range parameters were not. This condition broke winsync agreements
creation in ipa-replica-manage.
Make sure that ID range computation in DsInstance does not crash in
this cases so that winsync replica can be created. Also convert --binddn
option of ipa-replica-manage script to IPA native DN type so that
setup_agreement does not crash.
https://fedorahosted.org/freeipa/ticket/2987