adcli join with short name fails

When running adcli on a host where only the short name is set, adcli tries to add service principals twice (since short and DNS name are the same). The was a minor issue as each attribute was added on its own.

But since now all attributes are added in one run to allow a user with limited permissions to join a domain the join fails because LDAP does not allow to add the same attribute twice.

To avoid this it should be checked that the service principals are not added twice.

See https://bugzilla.redhat.com/show_bug.cgi?id=1649868 as well.