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
(cherry picked from commit 74eafa4b49dcf2f76b7627739d26e457fa1775db)
Merge "Bump minimum default RAM for Ironic nodes to 1GB" into stable/icehouse