mgr/cephadm: redirect browser to correct port by identity provider

mgr/cephadm: redirect browser to correct port by identity provider

After authentication, the external identity provider was redirecting
to the correct dashboard address but omitting the external port causing
the browser to redirect to the default https port (443) which isn't
used since an external port was configured in mgmt-gateway spec file.

The nginx external_server_conf.j2 file was changed to use $http_host
instead of $host in order for the oauth2-proxy service to correctly
construct the dashboard URL including the non-standard port.

Fixes: https://tracker.ceph.com/issues/74024
Signed-off-by: Yonatan Zaken <yzaken@redhat.com>