replace sgdisk subprocess calls with a helper

[alfredodeza]

This fixes issue 6979, but most importantly adds a couple of things to ceph-disk that should get used a bit more and hopefully used in the other ceph-* scripts.

• There was too much repetition for subprocess.* calls that were the same. By introducing a helper the repetition gets reduced and we now can handle errors for subprocess calls in one place.
• Adds 2 Exceptions, because having custom messaging in the error was needed (something that the existing ones did not provide)
• The try/except block at the end now just raises SystemExit vs. printing to stderr and then calling sys.exit(1). Raising that exception achieves the same functionality and it is just one call and easier to read.

This is how the error looks before these changes:

vagrant@node1:~$ ceph-disk list
Traceback (most recent call last):
  File "/usr/sbin/ceph-disk", line 2342, in <module>
    main()
  File "/usr/sbin/ceph-disk", line 2331, in main
    args.func(args)
  File "/usr/sbin/ceph-disk", line 2008, in main_list
    part_uuid = get_partition_uuid(dev)
  File "/usr/sbin/ceph-disk", line 1918, in get_partition_uuid
    stderr = subprocess.PIPE).stdout.read()
  File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1249, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

This is how it looks now:

vagrant@node1:~$ ceph-disk list
ceph-disk ExecutableNotFound: sgdisk not in path. Could not run command: sgdisk -i 1 /dev/sda

[liewegas]

we are just assuming that this matches the search path? would it make more sense to return the match and use it explicitly?

[alfredodeza]

Sure, that sounds reasonable