Bump minimum default RAM for Ironic nodes to 1GB

Set the default RAM for Ironic BM VMs to 1GB to ensure DIB produced deployment
ramdisks function correctly across older and newer kernels, regardless of
rootfstype.

Change-Id: Ifc15a4430aa63c3599e7fd76f5116dc7b728ead0
Closes-bug: #1311987
Merge "Bump minimum default RAM for Ironic nodes to 1GB"