Specify rootfstype=ramfs deploy kernel parameter

Specifies that the kernel use initramfs rather than tmpfs to back
the booted deploy ramdisk, eliminating the 1GB min. memory requirement
for Ironic nodes.

Change-Id: Ibbb0b9d4ce75697a7cbf2756cf4cb8323a56b137
Related-bug: #1311987
Merge "Specify rootfstype=ramfs deploy kernel parameter"