xDS interop: Python LB tests build and use the python server (#30637)
xDS interop: Python LB tests build and use the python server (#30637)

Undoes https://github.com/grpc/grpc/pull/27096.
While we lost context why py tests were used pinned cpp server,
we think this is due to lack of support of the set_not_serving RPC
in the python server, see https://github.com/grpc/grpc/issues/30635.

This RPC is only used in two tests, and for them we added a
temporary override of the test server to the reference Java server,
see https://github.com/grpc/grpc/pull/30636.

All other LB tests should work with the python server just fine.

jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,grpc/grpc,grpc/grpc,stanley-cheung/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,jtattermusch/grpc,grpc/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,stanley-cheung/grpc,stanley-cheung/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,jtattermusch/grpc,grpc/grpc,jtattermusch/grpc,grpc/grpc,jtattermusch/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,grpc/grpc,stanley-cheung/grpc