Use 1024 MB RAM for Ironic agent VMs

Our ramdisk is too heavy to run in 512 MB. This is a short-term
fix to get this working; long term goal is to reduce this, likely
by using a DIB build for CI.

Change-Id: I5a75368b55be1d206f54904dc807b4df1edc9314
Merge "Use 1024 MB RAM for Ironic agent VMs"