Refactor: use single ENV var for Encoding/Span types

[jan25]

Refer to
https://github.com/jaegertracing/xdock-zipkin-brave/blob/master/src/main/java/io/jaegertracing/xdock/zipkin/Application.java#L80 and
https://github.com/jaegertracing/xdock-zipkin-brave/blob/master/docker-compose.yml.

We have ENCODING and JSON_ENCODER to decide the v1 or v2 routes and the Span encoding type. These must be unified and only use a single ENV var.

Similar fixing need to be done in https://github.com/jaegertracing/jaeger/blob/master/crossdock/docker-compose.yml