mgr/cephadm: Fix mgmt-gateway default port in get_port_start()

mgr/cephadm: Fix mgmt-gateway default port in get_port_start()

The mgmt-gateway port was already defaulted to 443 in most places, but
get_port_start() did not apply this default. Since the output of
get_port_start() is used both to configure the daemon ports which are
later used to to open them in firewalld, this inconsistency meant the
HTTPS port was not opened when firewalld service was active.

This change makes get_port_start() also default to port 443, ensuring
the daemon is configured correctly and the corresponding firewalld port
is opened as expected.

Fixes: https://tracker.ceph.com/issues/74015
Signed-off-by: Redouane Kachach <rkachach@ibm.com>