cluster: Document the new UUID env variable
cluster: Document the new UUID env variable

In the previous commit I missed git adding this documentation to
explain the purpose of the new variable.

Signed-off-by: Antoni Segura Puimedon <532ff71c0f0c138e61afd0c77279be9f5bb6c4f0@midokura.com>

celebdor/midonet-docker