Lower default IRONIC_VM_SPECS_RAM

Revert the default required RAM for Ironic nodes from 1024MB to 512MB.
Ironic now passes a kernel parameter to deploy ramdisks that avoids use
of tmpfs and lowers minimum memory requirements.

Change-Id: I20226c496fdd06ecfc6769a60b6d6f82887130a8
Merge "Lower default IRONIC_VM_SPECS_RAM"