Add state attribute to StorageVolume

[allardhoeve]

1. Currently, volume state is defined by the driver in a key in extra. This key varies between drivers. Also, the value varies between drivers.
2. However, it is a commonly accessed attribute that should be unified across drivers, just as Node state is unified in NodeState.
3. This PR does exactly that.

Some caveats:

1. Except for OS and EC2, all other drivers return StorageVolume objects with state UNKNOWN.
   1. I have no access to GCE.
   2. I could find no exhaustive list of GCE disk states.
   3. The same holds for all the other drivers.
   4. I do not think this is a problem, because the instance variable does not currently exist at all 😉

UPDATE: GCE list is here: https://cloud.google.com/compute/docs/reference/latest/disks#resource

Acceptable values are:
"CREATING": The disk is being created.
"FAILED": The disk failed to be created.
"READY": The disk is ready to use.
"RESTORING": The disk is being restored from either a snapshot or an image.

[gertjanol]

EC2 Volume states: creating | available | in-use | deleting | deleted | error
OpenStack Volume states: creating | available | attaching | in-use | deleting | error | error_deleting | backing-up | restoring-backup | error_restoring | error_extending

[allardhoeve]

Fixed unknown node states, thanks @gertjanol

[allardhoeve]

@Kami, does this need a proposal @ dev?

[Kami]

Minor style thing - doing self.VOLUME_STATE_MAP.get(raw_state, StorageVolumeState.UNKNOWN) saves a couple of lines of code. On top of that, code also won't break and require an immediate update if a provider adds a new state (unlikely, but still).

[allardhoeve]

Done.

[Kami]

Need to update the changelog.

Besides that, LGTM, +1

[allardhoeve]

thanks!

[allardhoeve]

Documented the changelog. Will wait for the build, then merge.

[Kami]

Looks like the build is stuck :/

[allardhoeve]

Travis is very, very slow lately

[Kami]

Yeah, build just seems to be stuck. Probably best to just merge since since we don't have admin permissions and we can't restart the build (we could push a "dummy" commit but that's annoying...).