OpenAPI: Change openapi URLs

Recently, we have moved openapi implementation from sal-rest-docgen to openapi and we completely get rid of apidoc naming.

Reflect this change also in URLs, so for example:

1. http://localhost:8181/apidoc/explorer/index.html becomes http://localhost:8181/openapi/explorer/index.html
2. http://localhost:8181/apidoc/openapi3/apis/single becomes http://localhost:8181/openapi/api/v3/single
3. http://localhost:8181/apidoc/explorer/index.html?urls.primaryName=17830-sim-device%20resources%20-%20RestConf%20RFC%208040 becomes http://localhost:8181/openapi/explorer/index.html?urls.primaryName=17830-sim-device%20resources%20-%20RestConf%20RFC%208040
4. http://localhost:8181/apidoc/openapi3/apis/mounts/1 becomes http://localhost:8181/openapi/api/v3/mounts/1

In order to achieve this goal we have to modify  org.opendaylight.restconf.openapi.jaxrs.WebInitializer.