Skip test_dhcp_agent_scheduler tests if there is no DHCP agent available

Previously those tests were skipped only if there wasn't
"dhcp_agent_scheduler" API extension available.
But in some cases, like e.g. when Neutron is run with ML2/OVN backend
this extension is available (after [1] will be merged) but still in
typical deployment there is no Neutron DHCP agent deployed together with
ML2/OVN backend.

So those tests should be skipped not only if there isn't API extension
at all but also when there is no any DHCP agent available in the
deployment.

[1] https://review.opendev.org/c/openstack/neutron/+/897528

Related-bug: #2038655
Change-Id: I9ccb710c8dc32765962a18bee1771c86fe1aaacd
1 file changed