Add support to run a specific migration

server/startup/migrations/xrun.js

 if (RocketChat.Migrations.getVersion() !== 0) {
-	RocketChat.Migrations.migrateTo('latest');
+	RocketChat.Migrations.migrateTo(process.env.MIGRATION_VERSION || 'latest');
 } else {
 	const control = RocketChat.Migrations._getControl();
 	control.version = _.last(RocketChat.Migrations._list).version;