mgr/cephadm: mock get_fqdn in secure alertmanager config test

mgr/cephadm: mock get_fqdn in secure alertmanager config test

Before it was expecting "::1" when passed to get_fqdn to
resolve to "localhost" but in some environments it would instead
resolve to "localhost.localdomain" and the test would fail. This
test isn't testing the functionality of get_fqdn, so let's just
mock it so the test passes regardless of how get_fqdn resolves "::1"

Signed-off-by: Adam King <adking@redhat.com>