support more disk lookup keys

[dwlehman]

The current support in blivet does not include support for:

1. /dev/disk/ symlinks
2. WWN/WWID

[pcahyna]

you said in #35 (comment) that it supports /dev/disk/ symlinks. I just tried and this works:

      storage_volumes:
        - name: barefs
          type: disk
          disks:
            # /dev/vdc
            - /dev/disk/by-path/virtio-pci-0000:08:00.0
          fs_type: ext4
          mount_point: /mnt/barefs

A symlink to a partition does not work though.
I wonder why is it necessary to add support for block device types one by one? Can't the role just use any block device node or a symlink to it? Tools usually don't care what device type they are working on or whether it is a symlink: they can just open() the path and the kernel handles all for them.