Revert "fix validation error at detaching volume from the halted instance."

This reverts commit 5f385e228aa5fe27ff03dd9fe0c11d66bdb3697d.
force reload volume and vif object when the association from instance is updated.

the validations was getting wrong path when they run after add_ or remove_
association method calls. I was missing the consideration that the association object
is cached by Sequel.
Merge pull request #357 from axsh/fix-halted-instance-shared-vol

Fix volume attach operation failure after PR #355