qa/cephadm: query iSCSI gateway FQDN from inside the container
rbd-target-api validates that the gateway hostname supplied by gwcli
matches the container's own socket.getfqdn(). Running the same call on
the host can return a different value when the host and container resolve
names differently (e.g. on Rocky 10), causing gateway creation to fail
with HTTP 400 and all subsequent gwcli configuration to break silently.
Query the FQDN from inside the iSCSI container directly so the value is
always consistent with what rbd-target-api expects. This also removes the
"run twice" workaround, which was compensating for host-side DNS
warm-up flakiness rather than addressing the underlying mismatch.