AMQP-359: Reset amqp-log4j to default after test

JIRA: https://jira.springsource.org/browse/AMQP-359

Previously `AmqpAppenderIntegrationTests` configured `LogManager`
to use `log4j-amqp.properties`, who turned on DEBUG for `org.springframework.amqp.rabbit`
and all other tests generated a lot of DEBUG log message for their reports.
It causes a long time for parsing test results on CI.

Revert `LogManager` to use `log4j.properties` after `AmqpAppenderIntegrationTests`

**Cherry-pick to 1.2.x**