Vaults should only register up to collateral ceiling

[gregdhill]

Is your feature request related to a problem? Please describe.
Vault registration will fail when using the faucet if the amount exceeds the free balance or currency ceiling.

Describe the solution you'd like
The Vault should use the maximum possible value (i.e. max(ceiling - total_collateral, max(amount, free_balance))) to prevent exiting early.

Additional context
This may actually be preferable behavior since otherwise the operator will not know that the full amount was not locked.