Ensure cellar is owned by gravity contract before submitting cork

[cbrit]

It is required of cellar deployers to transfer ownership to the gravity contract via transferOwnership(address) before gravity will be able to execute functions that have the onlyOwner modifier (which should be all functions that Strategy Providers use). Since there currently is no automatic way to transfer ownership to gravity upon governance approval of a new cellar, Steward should check and then cache whether or not a target cellar address is indeed owned by gravity before submitting a cork. The owner() contract function will return the owner address.