fsdev_disks.get_disk_list(): Add option to get unpartitioned devices

fsdev_disks.get_disk_list(): Add option to get unpartitioned devices

The default behavior of the function fsdev_disks.get_disk_list()
is to return only partitions, rather than possibly unpartitioned
disks (ie, it would return /dev/sdc3 but not an unpartitioned
/dev/sdd). Let's add an option that makes all devices to be
returned, with defaults to the original behavior.

Also, turned a large chunk of content on part of the docstring
for that particular function.

Changes from v2:
* Handle scenario if particular disk have partitions, then it
skips unpartitioned device name and adds only partitioned
devices e.g. /dev/sdc have partitions /dev/sdc1, /dev/sdc2 then
only sdc1,sdc2 are listed and not sdc

Signed-off-by: Shrirang Phansalkar <sphansalkar@stec-inc.com>
Signed-off-by: Vyanktesh Yadav <vyanktesh.yadav@gmail.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
git-svn-id: svn://test.kernel.org/autotest/trunk@5493 592f7852-d20e-0410-864c-8624ca9c26a4