Price authority updated via governance in Vault Factory

[dtribble]

What is the Problem Being Solved?

The vault factory relies on a price authority, but the price authority implementation could change strategy, get updated, have bug fixes, etc. The two contracts could be more decoupled if the price authority for the vault were able to be changed by governance.

Description of the Design

Enable changing the price authority instance. All outstanding queries are only for liquidation; they could just be reissued if the price authority changed.

Security Considerations

Oracles are a complicated, separate system. Enabling them to be upgraded with minimal impact on teh vault provides bettersafety isolation and better upgrade support.

Test Plan

TBD