Think about adding missing provider when we have extra provider

[sudssm]

@DoronShapiro

consider making a system with a,b,c
then loading the system with a,b,d.
We load into readonly mode, and ask for c.

add c
Now we have a complete set! but d is actually blank.
On future operations, d will fail and go to yellow (but be repaired, if he is behaving).

We should probably detect if the filemanager's provider set is a superset of that stored in the manifest. And then... automatically reprovision? But the user may want to add more providers. Stay in ReadOnlyMode until the user reprovisions? But that overloads the meaning of ReadOnlyMode in an ugly way (since we can actually write, but we perhaps don't want to).