[CD13] Document using external AMQ brokers with CD images

This documents CLOUD-2225

I think it should be sufficient to document two scenarios:

Something like:

(1) One external broker:

oc new-app eap71-basic-s2i \
-p APPLICATION_NAME=eap71-mq \
-e MQ_SERVICE_PREFIX_MAPPING="broker1-amq=BROKER1_MQ" \
-e BROKER1_AMQ_TCP_SERVICE_HOST=broker1-amq-tcp \
-e BROKER1_AMQ_TCP_SERVICE_PORT=61616 \
-e BROKER1_MQ_USERNAME=test1user \
-e BROKER1_MQ_PASSWORD=test1pass

(2) Two or more:

oc new-app eap71-basic-s2i \
p APPLICATION_NAME=eap71$1 \
-e MQ_SERVICE_PREFIX_MAPPING="broker1-amq=BROKER1_MQ broker2-amq=BROKER2_MQ" \
-e BROKER1_AMQ_TCP_SERVICE_HOST=broker1-amq-tcp \
-e BROKER1_AMQ_TCP_SERVICE_PORT=61616 \
-e BROKER1_MQ_USERNAME=test1user \
-e BROKER1_MQ_PASSWORD=test1pass \
-e BROKER2_AMQ_TCP_SERVICE_HOST=broker2-amq-tcp \
-e BROKER2_AMQ_TCP_SERVICE_PORT=61616 \
-e BROKER2_MQ_USERNAME=test2user \
-e BROKER2_MQ_PASSWORD=test2pass

(Obviously the names could be improved).

The second case is very much an edge usage I think, so just as a note at the bottom of the section that has (1) would be sufficient, I think.