ipalib: Remove redundant and star imports from host plugin
Also fixes incorrect error catching for UnicodeDecodeError.
Part of: https://fedorahosted.org/freeipa/ticket/3979
Reviewed-By: Petr Viktorin <pviktori@redhat.com>
Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>