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
Adding multiple server support to nova reset-state

Adding support specifying multiple servers to nova reset-state in
a single invocation.

Change-Id: I7add0c26b0e59b31751de7a60ebdde3f568b3354
Closes-Bug: #1314217
Merge "Adding multiple server support to nova reset-state"