Add retry to symlink resolution for iscsi volumes

The ISCSI driver returns a device name that maps to
/dev/disk/by-path/ip-%s-iscsi-%s-lun-%s. This points shows up as a
symlink that points to the actual device in /dev that matches the
pattern of /dev/sd*.

Occasionally, this symlink does not resolve on the first pass which
causes the /dev/disk/by-path/ip-%s-iscsi-%s-lun-%s path to be returned
instead of the actual /dev/ path. This causes the _list_host_devices
from the base volume driver to return no volumes.

This patch forces the ISCSI volume driver to retry the symlink resolution
until it gets the /dev/ path it needs.

Change-Id: Ia6f255b4243de6dff689cdb446fd168075f96cdb
Merge "Add retry to symlink resolution for iscsi volumes"