Add support for identity tokens in client credentials store

Update unit test and documentation to handle the new case where Username
is set to <token> to indicate an identity token is involved.

Change the "Password" field in communications with the credential helper
to "Secret" to make clear it has a more generic purpose.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
Upstream-commit: ba0aa5311aa27fe77166f03d8bcc0174e2985913
Component: engine

components/engine/integration-cli/fixtures/auth/docker-credential-shell-test

@@ -8,8 +8,8 @@ case $1 in
 		server=$(echo "$in" | jq --raw-output ".ServerURL" | sha1sum - | awk '{print $1}')
 
 		username=$(echo "$in" | jq --raw-output ".Username")
-		password=$(echo "$in" | jq --raw-output ".Password")
-		echo "{ \"Username\": \"${username}\", \"Password\": \"${password}\" }" > $TEMP/$server
+		password=$(echo "$in" | jq --raw-output ".Secret")
+		echo "{ \"Username\": \"${username}\", \"Secret\": \"${password}\" }" > $TEMP/$server
 		;;
 	"get")
 		in=$(</dev/stdin)