Readd and make "Grab first disk" option for Cinder RAW Mode

We usually do not want Cinder to grab additional disks that appear
on the cinder node, as those might be locally mapped LUNs and other
things totally unrelated to Crowbar. Grab first disk only by default.
Merge pull request #112 from dirkmueller/allocate_first_disk

Readd and make "Grab first disk" option for Cinder RAW Mode