Add 2.55 schema & update flavor API in flavors_client

Compute microversion 2.55 adds 'description' in
flavor APIs response and new API to update the same.

This commit fill the schema gap for that and also
add new updte flavor API in flavors_client lib.

Change-Id: I91c049c3aa9dab5a272369edd18f2c5890d1e5b0
8 files changed