Fix rxtx_factor name for creating a flavor

On v3 API, rxtx_factor parameters has been renamed to
'os-flavor-rxtx:rxtx_factor' for consistent interfaces.
This patch applies it to 'nova flavor-create' command.

Closes-Bug: #1350022
Change-Id: I49e24f0220f87616f346f36973312b2f29333841
Merge "Fix rxtx_factor name for creating a flavor"