XDS environment configuration can be set using various methods.
Here is the list of various methods, ordered by priority:
1/ from command line option: "--config myConfig.json"
2/ from environment variable XDS_CONFIG
3/ $HOME/.xds/cli/cli-config.env file
4/ /etc/xds/cli/cli-config.env file
Change-Id: I1b59c05e583336fe05459de91a35d47cf5ddf670
Signed-off-by: Sebastien Douheret <sebastien.douheret@iot.bzh>