Fix the help messages to specify image/flavor name

User can specify flavor/image name as the arguments of "nova boot"
command instead of their ids, but the help messages does not explain
that. This fixes the help messages.

Fixes: bug #1211156

Change-Id: I47813c8ebbe5fc83d627f016e3751918b3ab11cb
Merge "Fix the help messages to specify image/flavor name"