update the host default price and collateral values

The price of siacoin has gone up substantially since the last release, meaning that the defaults are now very high. This adjusts the defaults down to more reasonable levels.

The defaults have also been adjusted a bit to help target a healthy, competitive network.

The collateral in particular is lower than what was previously recommended (even adjusted for the siacoin price change) because renters had been making more contracts than using them, zapping many hosts of all their collateral.

The lower collateral price should help the hosts get farther while the network bootstraps. We may need to reconsider some things if the renters continue to allocate more collateral than they use, but lazy contract formation should help substantially.