Added the option to define rabbitmq's queues as "durable"

Due to its nature, RabbitMQ data is very volatile.
Since messaging is all about speed and amounts of data,
all the messages are kept in RAM unless you define
queues as "durable", which will make RabbitMQ write them
to disk. This is actually supported by OpenStack with
the setting amqp_durable_queues=True in nova.conf.

Change-Id: I854970b8a6d5e771f8bd4410cb3bcf6bf705cee7
Implements: blueprint add-rabbit-durable-queues-param
Parameterize disk_cachemodes

The current manifests don't expose a way for end users to set
the disk_cachemodes directive in nova.conf, which can be useful
for improving disk performance.  This patch adds a parameter
allowing users to specify a list of values to be written in
to the disk_cachemodes setting.  The default value is an empty
list, which will cause the disk_cachemodes setting to be removed
entirely from nova.conf (thus matching the Nova default and
current behavior).

Change-Id: I9173ef79e4e573515f290734535907794dda7dbf
Merge "Added the option to define rabbitmq's queues as "durable""
Merge "Parameterize disk_cachemodes"