tools/setup_{RedHat,Debian}.sh can try to process multiple loop devices

I'm not quite sure how this happened, it would seem logical that it is due to running tools/setup_RedHat.sh multiple times, but i'm sure it failed on the first run.

Either way, it would be good if the script was idempotent. The losetup isn't safe from this and neither is the $DEV assignment.

Currently the parted command is passed multiple loop devices, which is failing.

Seeing this from the output:
{{
 sudo dd if=/dev/zero of=/docker bs=1M count=20480
20480+0 records in
20480+0 records out
21474836480 bytes (21 GB) copied, 48.1589 s, 446 MB/s
+ losetup -f /docker
++ awk -F: '/\/docker/ {print $1}'
++ losetup -a
+ DEV='/dev/loop0
/dev/loop2
/dev/loop3'
+ sudo parted /dev/loop0 /dev/loop2 /dev/loop3 -s -- mklabel msdos
Usage: parted [OPTION]... [DEVICE [COMMAND [PARAMETERS]...]...]
Apply COMMANDs with PARAMETERS to DEVICE. If no COMMAND(s) are given, run in
interactive mode.

OPTIONs:
  -h, --help displays this help message
  -l, --list lists partition layout on all block devices
  -m, --machine displays machine parseable output
  -s, --script never prompts for user intervention
  -v, --version displays the version
  -a, --align=[none|cyl|min|opt] alignment for new partitions

COMMANDs:
  align-check TYPE N check partition N for TYPE(min|opt) alignment
  help [COMMAND] print general help, or help on COMMAND
  mklabel,mktable LABEL-TYPE create a new disklabel (partition table)
  mkpart PART-TYPE [FS-TYPE] START END make a partition
  name NUMBER NAME name partition NUMBER as NAME
  print [devices|free|list,all|NUMBER] display the partition table, available devices, free space, all found partitions, or a particular partition
  quit exit program
  rescue START END rescue a lost partition near START and END
  rm NUMBER delete partition NUMBER
  select DEVICE choose the device to edit
  disk_set FLAG STATE change the FLAG on selected device
  disk_toggle [FLAG] toggle the state of FLAG on selected device
  set NUMBER FLAG STATE change the FLAG on partition NUMBER
  toggle [NUMBER [FLAG]] toggle the state of FLAG on partition NUMBER
  unit UNIT set the default unit to UNIT
  version display the version number and copyright information of GNU Parted

Report bugs to <email address hidden>
+ sudo rm -rf /var/lib/docker
+ sudo mkdir /var/lib/docker
+ sudo mkfs.btrfs -f /dev/loop0 /dev/loop2 /dev/loop3
btrfs-progs v3.19.1
See http://btrfs.wiki.kernel.org for more information.

Performing full device TRIM (20.00GiB) ...
Turning ON incompat feature 'extref': increased hardlink limit per file to 65536
Turning ON incompat feature 'skinny-metadata': reduced-size metadata extent refs
skipping duplicate device /dev/loop2 in FS
skipping duplicate device /dev/loop3 in FS
not enough free space

}}

This is caused by:
{{
# DEV=$(losetup -a | awk -F: '/\/docker/ {print $1}')

# echo $DEV
/dev/loop0 /dev/loop2 /dev/loop3

# losetup -a
/dev/loop0: [64768]:397121 (/docker)
/dev/loop2: [64768]:397121 (/docker)
/dev/loop3: [64768]:397121 (/docker)
}}