Take iSCSI deploy out of pxe driver

This commit aims at separating the iSCSI deploy portions
from pxe driver so that it can reused in the newly proposed
iLO virtualmedia driver.

Implements: blueprint ironic-ilo-virtualmedia-driver
Change-Id: Id0a4b0fab797c884448a27b1cd5e3a1da05786a7
Add support for creating vfat disk images

This commit introduces support for creating temporary
small vfat images in ironic which will be used for
passing information to baremetal node using virtual
media through OOB channel.

Change-Id: I42b88da02090aec310f567642d3174d0f482b585
Implements: blueprint ironic-ilo-virtualmedia-driver