Add $OMT_HOST support to make start-postserve

[zstadler]

When $OMT_HOST is not localhost, say 192.168.99.101, the json file served by make start-postserve at http://192.168.99.101:8090 has

"tiles": ["http://localhost:8090/tiles/{z}/{x}/{y}.pbf"]

rather than

"tiles": ["http://192.168.99.101:8090/tiles/{z}/{x}/{y}.pbf"]

In such an environment, make start-maputnik prints following instructions:

***********************************************************
*
* Start maputnik/editor
*       ---> go to http://192.168.99.101:8088
*       ---> set data source / TileJSON URL to http://192.168.99.101:8090
*
***********************************************************

However, maputnik cannot find the tiles using the localhost address included in the TileJSON URL - http://localhost:8090/tiles/{z}/{x}/{y}.pbf