Update nova boot help

Actual help string for --block-device flag is not clear.
User doesn't know which parameters are required and which are not.
Change improves description of each parameter, so that user can
understand how the --block-device works.

Change-Id: If23cb2d64adb51dfb45e48511ca25988bc43b739
Implements: nova boot --block-device usage unclear
Partial-Bug: #1323775