Fix to allow compute driver attributes

Add the following compute driver attributes
- default_ephemeral_format
- preallocate_images
- use_cow_images
- vif_plugging_is_fatal
- vif_plugging_timeout

Change-Id: Icd014c1c88ab2d4c6773a4cbe14b638c2332fd4a
Closes-Bug: #1317564
Fix to allow nova workers to be overrideable attributes

Current code has workers hardcoded. Nova already defaults workers
to number of available cpus.  No way for administrator to customize
number of workers.

Change-Id: If31e1eb9c743646ff471a6fcd6fe15b03f289cc3
Closes-Bug: #1317247
Add missing AMQP options to nova.conf

Use the new attributes defined in common to allow
these extra MQ settings to be set for compute.

Depends on change I850c01a40aab02cf4531f7983543ef89e0e60446

Change-Id: I6adf9ff6c35b05294d58e8e6dac5d045e0d0d70f
Remove superfluous else branches

These "variable = nil" initialisations happen automatically.  While its
not much that is gained from that now, let's not create bad examples to
follow.

Change-Id: I48134f418a691e95679e95e742c31f8b13f3f79d
Merge "Remove superfluous else branches"