Make id option overwrite XDS_xxx_ID env var

Make id option overwrite XDS_xxx_ID env var

The behavior must be the same when XDS_SDK_ID is defined and user
set -id option or simply give an id without setting "--id" string
option.
IOW all following commands must give the exact same result  (get info
of sdk 9d69):
./bin/xds-cli sdks get -id 9d69
./bin/xds-cli sdks get 9d69
XDS_SDK_ID=9d69 ./bin/xds-cli sdks get
XDS_SDK_ID=1234 ./bin/xds-cli sdks get -id 9d69
XDS_SDK_ID=12345 ./bin/xds-cli sdks get 9d69

Change-Id: Idbd3f052df8172d56de98f827743b43d70d37393
Signed-off-by: Sebastien Douheret <sebastien.douheret@iot.bzh>