check backup object's state before destroying instance.
check associated instance state before destroy ssh key pair.

It allows to leave association between instance -> ssh_key_pair table.
Merge pull request #299 from axsh/localvolume-test-scenario

mussel: add backing up disk-full volume.
Merge pull request #300 from axsh/fix-destroy-validation

validation at destroy instance
mussel: fix backup volume target from boot-volume to ex-volume.