v2 api - return bootable attr value on volume list

Add bootable to the list of values returned by display_list.  This
was returned in v1 api, and is still in v2 header, but was missed
in the new implementation.

Closes-Bug: #1232287

Change-Id: If7460b1c8ab4af417117c4bf6cfdccc5fcf21f46
Merge "v2 api - return bootable attr value on volume list"