Put rabbitmqadmin.conf in /etc/rabbitmq

This commit puts rabbitmqadmin.conf in /etc/rabbitmq instead of /tmp so
that puppet does not have to replace it if /tmp gets cleared. It also
forces the rabbitmq_exchange provider to call rabbitmqadmin with
-c /etc/rabbitmq/rabbitmqadmin.conf on both destroy and create so that
both operations have access to the contents of the config file.