deploy element: Call find_disk in loop w/ timeout.

Updates our deploy element's init script so that it calls
find_disk in a loop until it returns true (meaning we have a disk).

Previously on some bare metal machines find_disk would return ""
which causes the subsequent iscsi target commands to fail due to
a bad backing store.

This commit also updates the error message when start_iscsi_target
fails to be a bit more accurate as well.

Fixes LP Bug #1190984.

Change-Id: I3cd535d6672c197c1c3c539c83bba36be7a14e18
Merge "deploy element: Call find_disk in loop w/ timeout."