Use the amqp_ prefix for AMQP variables

Currently we use the rabbit_ prefix for ceilometer_collector and the amqp_
prefix for the remote_collector. This commit changes rabbit_ to amqp_ to
use the same variable names everywhere.
diff --git a/README.rst b/README.rst
index 6d463cb..2aaef14 100644
--- a/README.rst
+++ b/README.rst
@@ -155,10 +155,11 @@
         influxdb_time_precision: ms
         influxdb_username: lma
         resource_decoding: false
-        rabbit_host: 172.16.10.253
-        rabbit_port: 5672
-        rabbit_queue: metering.sample
-        rabbit_vhost: /openstack
+        amqp_exchange: ceilometer
+        amqp_host: 172.16.10.253
+        amqp_port: 5672
+        amqp_queue: metering.sample
+        amqp_vhost: /openstack
 
 Default values:
 
@@ -169,9 +170,10 @@
 * ``influxdb_time_precision: ms``
 * ``influxdb_timeout: 5000``
 * ``poolsize: 100``
-* ``rabbit_port: 5672``
-* ``rabbit_vhost: /openstack``
-* ``rabbit_queue: metering.sample``
+* ``amqp_exchange: ceilometer``
+* ``amqp_port: 5672``
+* ``amqp_queue: metering.sample``
+* ``amqp_vhost: /openstack``
 * ``resource_decoding: false``
 
 Read more