Move ipmitool _set_boot_device to VendorPassthru

_set_boot_device must be exposed via vendor_passthru, allowing
operators to manually change it, and force to boot from net when
performing a pxe deploy.
Until now, it was just a private method of the power driver.

Partial-Bug: #1264596
Change-Id: I174f0a058bfe6276b7a42291661f3fd00de9a7ca
Merge "Move ipmitool _set_boot_device to VendorPassthru"