Flavor specs: Add rationale for encoding info into names

Created by: garloff

While we encourage to use the short names in the SCS flavor naming, we do have the capability to encode a lot of details into the flavor names. This is useful, as we may have (large) clouds, where customers have the ability to chose between very finegrained flavor types for selecting capabilities and we need to do this by chosing the flavor names. (The mechanism to use extra_specs and match it with images is broken -- requiring custom images just to be able to select the right VM flavor is a concept broken by design IMVHO.) We should clarify this in the docs.