Install 'ipmitool' package for Ironic

Ironic's reference / recommended driver is the "pxe_ipmitool" driver,
which requires the 'ipmitool' package and system binary. Thus,
devstack should install it.

Also, Ironic will soon be validating that ipmitool is present in the
system path when loading the pxe_ipmitool driver, as it does for other
drivers (seamicro, pyghmi) that have external dependencies.

Causing devstack to install this package when Ironic is enabled will
also help users who wish to test Ironic against physical hardware that
has IPMI support, without having to perform additional manual steps
(like enabling the pxe_ipmitool driver and installing the ipmitool
packge by hand).

Change-Id: I13da5f8978b280b192f9b033c348a11bfc44c9d2
Merge "Install 'ipmitool' package for Ironic"