Fix flavor extra_spec usages

"nova-manage" command is deprecated and "nova flavor-key" can set
flavor extra_spec instead. This patch changes the usage to show
"nova flavor-key".
In addition, "nova flavor-set" command does not exist.
This patch fixes it with "nova flavor-key" command.

Change-Id: If41899cf0af463ccc2db5127d53398f73e3a26a7