Fog::Model#wait_for: eliminate inner retry loop

AWS frequently takes more than 3 seconds for an instance to appear.
Rather than having the overlapping timeouts for reload and
instance_eval, simply make them both occur with the same timeout.

 + Prevents spurious failures when waiting for AMI
 + Less code
 + Easier to follow flow

 - Need to consider whether it was important to raise Fog::Errors:Error
   distinct from the block failing.