REGTESTS: mcli: check the master and worker unique identifiers

REGTESTS: mcli: check the master and worker unique identifiers

The test forces the worker's ID from the configuration so that it can be
matched exactly, both from the "worker_id" sample fetch function, from
$HAPROXY_WORKER_ID and from "show info", while the master's one, which is
generated, is matched against the UUID version 7 format, in the master's
environment, in "show info" and through the "master_id" sample fetch
function.

A second instance runs without a master and lets its worker ID be
generated, which permits to check that "master_id" then finds nothing and
that the generated identifier matches its exported copy.

Both instances also verify what "stats show-node" reports on the HTML
page, with and without a master.