Add filter mechanism to /api/5/gateways

Filtering works for /api/5/gateway:

pea@peabox:bootstrap curl -s 'localhost:8443/api/5/gateway?cc=abc' |jq                                               
"CountryCode not supported"                                                                                                                                                                                         

pea@peabox:bootstrap curl -s 'localhost:8443/api/5/gateway?cc=FR' |jq                                               
...valid json

It does not work using the /api/5/gateways endpoint:

pea@peabox:bootstrap curl -s 'localhost:8443/api/5/gateways?cc=abc' |jq
[                                                                                                                      
  {                                
    "experimental": false,   
    "healthy": true,           

The difference between the endoints is explained here: #62 (comment 1224635)