diskpart not mapped for cobbler

Map install_drive as an array to diskpart so cobbler generates preseed
files which target an install drive or install drives.

Change-Id: I86c40009ea367bd797856b3754d7347e9ca1dd2c
Closes-Bug: #1325064
Merge pull request #90 from CiscoSystems/bugs/1325064

diskpart not mapped for cobbler