Simplify cli plugin config file entry

Make it a simple `map[string]string` for now.

Added a unit test for it.

Signed-off-by: Ian Campbell <ijc@docker.com>

e2e/cli-plugins/help_test.go

@@ -13,7 +13,7 @@ import (
 
 // TestGlobalHelp ensures correct behaviour when running `docker help`
 func TestGlobalHelp(t *testing.T) {
-	run, cleanup := prepare(t)
+	run, _, cleanup := prepare(t)
 	defer cleanup()
 
 	res := icmd.RunCmd(run("help"))