Merge pull request #290 from axsh/localvolume-test-scenario

mussel,test: make sure to delete backup_object after backing up object.
fix epel installation
mussel: change "account_id" parameter policy from requried to optional.
Merge pull request #291 from axsh/feature-mussel-account-id-less-api-call

mussel: change "account_id" parameter policy from requried to optional.
mussel: add first local-volume-instance scenario.