test_cert_plugin: use only first part of the hostname to construct short name
The short hostname construction for the negative test case in
test_cert_plugin::test_cert_find suite could not work when domain name was
different as hostname of the test runner, leading to test failure. A more
naive approach works better in this case.
https://fedorahosted.org/freeipa/ticket/5688
Reviewed-By: Martin Basti <mbasti@redhat.com>