Handle missing price authority from AMM

[mhofman]

The vault task setup requires price authorities for the pair of loadgen token / stable token.

Currently we retrieve the price authority from the AMM, however the SDK is looking at dropping that feature. The loadgen should switch (or fallback) to a manual or fake price authority. A fake price authority may be sufficient for now given that the vault task doesn't currently attempt to exercise vault liquidation. The logic from demoIssuers should be sufficient.

[mhofman]

AMM has now been excised in Agoric/agoric-sdk#7074, requiring to disable the vaults task.

Once we switch to a new price authority, we can re-enable the vaults task in the integration tests of both this repo and agoric-sdk.