Add volume attachment status

[andrewgross]

Adds volume attachment status for filtering.

Current possible statuses are attached or detached, the intermediate states (attaching and detaching) are not implemented yet, because I couldn't figure out a good implementation. The detached status is tricky to test, since you have to keep the VolumeAttachment from detach_volume.

[coveralls]

Coverage increased (+0.001%) to 95.033% when pulling b417605 on andrewgross:feature/add-volume-attachment-status into 1f618a3 on spulec:master.

[spulec]

thanks!