Actions :: Update ActionTag with UUID structure

Now that Actions use a UUID as the ID; update ActionTag's to better
represent the Action.
Use strongly typed UUID's

    Also use utils.IsValidUUIDString
    Updates: ActionTag, EnvironTag
EnvironTag is not ready for strongly typed UUID

    This reverses 891e2be8c81f374ea745f449a5d766493e96cd3d for
    the EnvironTag.

    See http://paste.ubuntu.com/9149684/ for the issues with
    default initialized EnvironTag's (all zeros).
Merge pull request #32 from johnweldon/0-action-tags-cleanup

Actions :: Update ActionTag with UUID structure
Introduce DiskTag

DiskTag will be used to uniquely identify disks (block devices),
either attached or unattached. DiskTag's format is disk-<seq>.