Document workaround for AMQP reconnects in release notes

AMQP reconnects bug (LP1371723) added to known issues in the release notes,
workaround (install oslo.messaging from 5.1.1) documented.

Change-Id: I06a0db362ef58308bad81a65a529d9a32f38f492
(cherry picked from commit 3b67f9939b946dfe99046e5180955627159298f7)