Eager load roles and proposals for NodesHelper

In certain cases, it makes sense to eager load all roles/proposals.

On my setup, a loading a single chef object takes ~ 20ms, while fetching all
roles/proposals takes ~ 100ms (There are 62 / 32 of them in the db.)

So it seems that anything above 5 records should be faster with an eager
load. That of course assumes that loading everything at once won't slow
down too much with more records in the DB.

Feel free to independently verify/refute this!

Use filtering from pre-fetched records and cache the collection of all
records from the db.

This reduces the admin node show view load time from 900 ms to roughly
half.
Eager load proposals in the barclamp index view