[AWS|ELB] policy model update

Updates the ELB policies collection to be initialized with the result of calling `DescribeLoadBalancerPolicies` instead of relying on the limited data in `DescribeLoadBalancers`. The policies are cached until the ELB is reloaded.

I also discovered that when creating a public key policy AWS accepts a cert *or* the public key for the cert, but always returns the public key. To simplify the tests I am always passing the public key of the cert.

Ref fog/fog#2014