rabbitmq-server: Put files in $HOME by default

Before, files were put in /var, requiring the server to be run as a
privileged user even when just testing locally. This can be overridden
by setting the SYS_PREFIX env variable, or on a more coarse-grained
basis in /etc/rabbitmq/rabbitmq-env.conf

Signed-off-by: Shea Levy <shea@shealevy.com>