[services] Tunnelbroker - Add messages table name to ConfigManager.

Summary: Tunnelbroker needs to store messages in the DynamoDB database. We need to add ability to change the database name from the Config file and add the default name for it.

Test Plan:
Get the config option using:
ConfigManager::getInstance().getParameter(ConfigManager::OPTION_DYNAMODB_MESSAGES_TABLE);

Reviewers: karol-bisztyga, palys-swm, ashoat, jimpo

Reviewed By: karol-bisztyga, palys-swm, ashoat, jimpo

Subscribers: benschac, ashoat, palys-swm, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3116