The new method to get service names fails if dirsrv is down

[rcritten]

When a service name needs to be looked up in LDAP and dirsrv is not running then healthcheck returns an exception:

ipalib.errors.NetworkError: cannot connect to 'ldapi://%2Frun%2Fslapd-IPA-TEST.socket'

It should be more robust.

This is seen as a test failure in ipatests.test_integration.test_ipahealthcheck.py::TestIpaHealthCheck::test_ipahealthcheck_topology_with_ipactl_stop

[rcritten]

The reproducer is ipa-healthcheck --source ipahealthcheck.meta --check krb5kdc

[rcritten]

A similar traceback is seen when dirsrv is stopped with:

ipa-healthcheck --source ipahealthcheck.ipa.topology

[rcritten]

This is IPA issue https://pagure.io/freeipa/issue/9478