--secure was moved to front since many languages need flags to precede
positional parameters, and we'd like other languages to use the same
flags when feasible.
:8000 was removed from xds: target in the README, as it isn't all that
useful and is confusing as xDS itself provides the backend port numbers.
While most languages support setting environment variables during runtime,
Java does not. In Java, the preferred approach is to use Java System Properties
in order so specify configuration options. By checking for the existence
of the io.grpc.xds.bootstrap property if GRPC_XDS_BOOTSTRAP is not found,
it is possible to either supply the bootstrap location during runtime or as
a Java argument. The environment variable still takes precedence in order
to not break any existing documentation.