Add 'bootable' field to volume show response

The cinder volume data model shows a 'bootable' field as type boolean
and apparently it's coming back on the cinder API response when getting
a volume, but it's not in the API docs. This patch add the missing
'bootable' field to the sample response.

Change-Id: I3b1532faf4108fc08cd7695b2d8e5e90786c2a2e
Closes-bug: 1329762
Merge "Add 'bootable' field to volume show response"