Update DFI DUSD vault collateral requirement rules

[dcorral]

What kind of PR is this?:

/kind feature

What this PR does / why we need it:

Force 50% DFI collateral on vaults with DUSD loan

[BenTStark]

If you want to mint dTokens and DUSD, you need two vaults to get the same effect as before. Wouldn't it be easier from a user's perspective to evaluate Loans and Collaterals in mn_checks.cpp?
DUSD Loan has to be backed fully by DFI. Then all other loans must be backed by DFI and DUSD.

[kuegi]

If you want to mint dTokens and DUSD, you need two vaults to get the same effect as before. Wouldn't it be easier from a user's perspective to evaluate Loans and Collaterals in mn_checks.cpp? DUSD Loan has to be backed fully by DFI. Then all other loans must be backed by DFI and DUSD.

you do not need 2 vaults. DUSD loan doesn't have to be backed fully by DFI. Its just that as soon as you have DUSD loans in the vault, 50% of your required collateral needs to be DFI.
Only reason to have two vaults is if you want one 100% DUSD vault where you mint dToken, and one (smaller, otherwise you can combine them) vault with DFI where you mint DUSD. But then I ask the question why you would mint DUSD in the first place?