include context traits when looking up for a compatible context

[lburgazzoli]

Fixes #649

[nicolaferraro]

Fair enough. Traits defined on context may also be used to change the deployment behavior only, so it's not always necessary to exclude mismatching context. But I think it's safer to do so. 👍

[lburgazzoli]

This applies to platform and external contexts only, it may be restricted to platform created ones if needed but in any case I think automatic selection should go through the safer path and if needed user can explicit set the context to use.