Not invoke consumers that relay on not configure connections

[vkublytskyi]

Description

Cron should not try to invoke consumers that rely on AMQP if store administrator decided not to configure AMQP as this is an optional feature.

Fixed Issues (if relevant)

• Cron run failure on default installation #28

Manual testing scenarios

• Install Magento and run bin/magento cron:run

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)