Determine necessary volume changes as part of a deploy - fixes 272

Fixes #272.
There appears to be missing branch coverage here: http://build.flocker.hybridcluster.net/results/determine-volume-changes-272/flocker-1129/complete/flocker_node__deploy.html#n309

Perhaps a test for the case for an application which no-longer has an attached volume or a previously volumless application which now has a volume. 

(do we support those scenarios yet?)